Online marketplaces are typically valued by revenue, community engagement and potential. What is the company's current growth, what is the rate of growth, what is the market share potential, how it is the market, and what are the opportunities that the company affords. These all play a part in valuation. What is the reach? How many subscribers, users, etc all play a part

1) Number of offerings in their catalog
2) Number of active buyers
3) Number of buyers
4) Website traffic
5) Monthly gross sales
and so on.....

Just like any other business, brand and years in business matter as well.

Hope this helps.

Here are the most important factors for analyzing the efficiency of your marketplace.

Data for service providers

Number of buyers per day/month/year
Activity tracking per day/month/year
Customer liquidity (probability that a visit will lead to transaction)
Profit by countries/cities
Repeat purchase ratio
Revenue from buyers
Number of orders per buyer
Analysis of customer segments
Market conditions and trends

Data for marketplace owner

Number of listings on the marketplace
Listing growth rate
Provider liquidity (percentage of listings that lead to transactions)
Total sales value of the products or services sold via your marketplace
Tracking the steps user takes before buying
Customer acquisition cost (how much money you spend on new customer e.g. customer support or paid ads)
Seller growth rate
Revenue by location, market, acquisition channel or time
Take rate (percentage of the Gross Merchandise Value retained by your marketplace)

There are many online marketplace businesses popping up, but it can be hard for investors to know what they are worth. We all know valuation is an art not a science. We also know investors love marketplaces, and for good reason. Marketplaces can be exceptional business models. There are several metrics investors look for when valuing a marketplace, but here are the most important.
1. GMV, Gross Merchandise (or Market) Volume: GMV is the Holy Grail of marketplace metrics. In the first few years of a marketplace’s life, monthly GMV can be jagged, but up and to the right over the course of a full year is a positive trend. A marketplace is one of the hardest businesses in the world to start — you are trying to attract two sets of customers (supply & demand), often with opposing views, from a cold start. As a result, in the beginning growth looks like a jagged saw on a monthly basis until the flywheel starts spinning. As the marketplace’s network effects kick in, monthly growth becomes more consistent. Marketplaces, particularly pre-IPO marketplaces, are typically valued off of a multiple of run-rate GMV (i.e. existing month x 12). For our company’s fundraising rounds, we have gathered data on marketplace comps and found the valuation multiple is often 1-2x the run rate GMV, with an average of 1.4x and a median of 1.3x.
2. Revenue: True marketplaces charge a commission (or "take rate"). Typically, the commission is charged on the supply side rather than the demand side. Supply is, for example, the drivers in Uber’s case, or the hosts on Airbnb. The best marketplaces in history – eBay, OpenTable, Uber, Airbnb, and Lending Club all charge a commission. The challenge for a true online marketplace is to set a take rate that promotes both supply and demand and allows the marketplace to thrive. Finding that optimum take rate is critical since a take rate that is too high can spell trouble over the long term. Circle Up is an online private equity marketplace that connects consumer businesses to investors. When we initially set our take rate, for example, we recognized that the offline alternatives (small, regional investment banks) often charged 8-12% cash, plus 2-4% warrants, plus a monthly retainer, to raise equity for a company. These comps gave us a lot of space to set our own take rate aggressively lower. Investors should look for a healthy take rate, but not one that is so high it allows competitors to undercut on price.
3. Customer Acquisition Cost (CAC) for Supply: Supply is typically the harder side of a marketplace to acquire, and so the amount that is spent to ramp up supply is an important metric. For Lending Club, Prosper and SoFi it is the cost to acquire a borrower. Uber and Lyft are fighting ruthlessly over drivers. And OpenTable had to work hard to acquire restaurants and lock up their participation in the marketplace. In online marketplaces, customer acquisition cost is the cost to acquire supply. This is true whether that supply is borrowers, drivers, restaurants or, in the case of private equity platforms, companies selling equity. The best investors look for a healthy CAC relative to Long-Term Value (LTV). If you are overpaying on CAC relative to LTV, is there good reason? Is there a reason to believe the LTV/CAC ratio will normalize? (Investors typically look for 3-5x.) PayPal famously paid up for growth in its early days, paying $20 in cold hard cash for every new user: $10 to each new user and $10 to the user who referred them. Users could immediately spend or withdraw this balance. No strings attached. The company spent tens of millions on this promotion to grow their user base. And it worked. User growth went exponential: 7-10% a day. PayPal eventually phased this program out, dropping the bonus to $5 before ultimately stopping the program entirely when the marketplace reached critical mass.
4. Network Effects: There are a lot of ways to demonstrate network effects, and a healthy marketplace should be able to point to several. Fundamentally, if a marketplace has network effects then, when another participant joins, each participant receives more value from the marketplace. Think of how much value each restaurant and diner had when there were only a handful of restaurants on OpenTable. How valuable to diners or the restaurants could the platform possibly be? Now that there are several million, it is clearly a much more valuable marketplace to all participants. In our industry, private equity platforms, we find various indicators of network effects. They include rising quality of companies on the platform; lower marketing expenditures relative to GMV; more investors—the demand side—as the supply of companies grows; and faster and larger investment cycles as the platform grows.
5. Barriers to entry: As a marketplace grows, it becomes a stronger competitor, particularly to upstarts. Starting a ride service, for example, would have been far easier 10 years ago in San Francisco than it would be today with Uber and Lyft as competitors. Because of the network effects realized by Uber and Lyft, it would be next to impossible today to attract drivers or riders to a new entrant in the Bay Area.

There are other barriers to entry that add value to a marketplace:
1. Scale: How hard is it to build scale? In some markets it is difficult, but in others there are still opportunities to grow rapidly. In online lending, for example, the cost to create the evaluation algorithms is extraordinarily low. In addition, there is a lot of unmet demand so building scale is still relatively easy. You see that with nascent platforms getting scale quickly – the result will help drive down margins for all industry participants.
2. Brand: Just as a dollar value can be placed on the Apple or Coke brand, online marketplace brands have value that can be estimated based on several factors. Kickstarter has a strong brand and is the place to fund creative projects. Projects raising funding elsewhere have presumably been passed over by Kickstarter and so are perceived as less valuable trying to generate support on a second-tier crowdfunding platform. The premier marketplaces protect their brand by being selective while maintaining strong demand and supply. A brand can also be valued based on the expertise of the team operating the marketplace and, on the investors, backing the marketplace. (A quick look at Crunchbase will tell you who’s backing who.)
3. Regulatory barriers: Most successful marketplaces have dealt with regulators at one point or another. Those that engage effectively with regulators tend to become massive. Uber, Lending Club and Airbnb have all confronted regulatory challenges head on. Those that ignore regulators or try to go around them tend to lose out eventually. I predict, for example, that the days are numbered for those online equity marketplaces that are trying to avoid charging commissions to get around broker-dealer rules. Those businesses will either exist as online VC firms that are undifferentiated and add little or no value for investors or companies raising capital, or they will be forced to comply with regulations and modify their business model, leaving them to recreate themselves and play catchup to well-established online private equity marketplaces. Several of those platforms also struggle to raise capital for their own businesses because the smartest investors have realized the non-registered platforms are in trouble over the long-term.
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath