Turns out, although tackling the technical side of building a marketplace is not that easy, it is perfectly doable. The real challenge is getting people to use such marketplaces. It takes a lot of non-technical effort to build a marketplace. There's the obvious chicken-egg problem for most marketplaces. It is solvable, but usually requires a lot of capital to attract a lot of people, or the founder should have some sort of secret (being already established as an offline commissioner in a market, having the right connections, having a following and so on) so it is not easy to compete with them.
2 things I learned irrelevant to the business are these:
1) Looking back, I've made some serious mistakes, but I had no idea that these were mistakes at the time. Therefore, I must be making all kinds of mistakes today, which will only become apparent in the future, and that is actually a bit worrisome.
2) Prior to this, I've not experienced 'being a failure' in any sort of activity so strong, and it is not a good feeling.