Which side do I work on first- do I concentrate on getting buyers signed up and then sellers or vice versa? How do you do it
You'll want to target the side that has bigger pain as your entry point.
In addition, it's also worth considering a narrower subset in the "pain" category. One where you can imagine the onramp process.
An example would be airline reservation system (API). There are ticket buyers and customer service representatives. If you imagine the customer service representatives to have more pain (i.e. a poor experience and lack of features), then you would want to start with that side. A subset of the industry may be smaller regional or budget airlines. They may be incentivized to try something new based on cost concerns and a smaller overall footprint for their products.
You want to build a pain killer, not a vitamin: https://gettacklebox.medium.com/customer-interviews-so-you-ve-decided-to-take-your-startup-seriously-65715f4e9aec