We currently run an existing B2B services business, so have theoretically built up both sides of a 'marketplace' over 8 years. And now were thinking about changing our business model to a marketplace model. Given that's in place and we can see marketplaces in closely related markets operating, how much validation should we do?
First of all - good luck.
Second, I ALWAYS advise on doing market research and validation. The ratio of time versus value is always in favor of doing validation.
I've successfully helped over 300 entrepreneurs and would be happy to help you if you need. Just send me some background info by email before, so that the call itself can be 100% dedicated to giving you my advice (maximum value for your money)
You can see a more detailed answer about how to do market research/validation in one of my previous answers: