How do I validate the idea that it can be something consumers and vendors want? How can I make this company work?
Marketplaces can be wonderful but they are a challenge as you face Chicken V Egg.... you need supply and you need demand. Typically supply (product/ services) are easier if you keep the 'cost' really low (time, effort, content, set up, charges) as they are all looking for new sources of revenue and customers... the real challenge is CUSTOMERS.
If you could wave a magic wand and be successful at just one of these - I would pick customers... as IF you have customers the vendors will be there..
So focus on the customer - why they need or want or find value in your decor marketplace vs other options. And if you figure that out then next is thinking thru a strategy to make customers aware you even exist (as you now have a great value proposition). In todays world best (especially local) is if you can get a customer or anyone who likes what you are doing to share (socially).. could be something they saw or something fun etc...
Bottom line: figure out if the consumer wants, needs would find enough value in your concept - if you figure that out then the vendors will line up. Then the only last part to look at is the value equation - can you create enough value to consumers and vendors to enable you to BUILD and maintain a successful ($) business (IE: consumers or vendors may love it but enough to pay a price to fund your efforts)