In addition to that, they also:
- have mechanics to answer questions (for example, it's sometimes not obvious if a bike can take panniers),
- try to figure out if any of the bikes are stolen beforehand,
- deal with actually selling the bike so the seller doesn't need to be there the whole day (they just drop off the bike in the morning and pick up money or bike in the evening),
- have some sort of setup where you can take one of the bikes for a test ride.
In general it seems to me that a large part of the reason why that feels very nice (from a buyer's perspective at least) is that they remove incentives to be dishonest, so that everyone taking part can just be calm, trusting, and assume the same of others and won't be harmed by that. On the other hand, I think that a large reason why this became a major thing (there are nonspecialized flea markets and you can buy/sell bikes there too) is in large part that it's very hassle-free for sellers.
Regarding the place, Zurich has a few squares in the city center, which tend to get used for such things (I'm actually not sure how that works formally tbh). Sadly that's probably one of the larger problems with setting something like that up in the US (or am I very off?).
To give you a order of magnitude estimate, there is one such flea market organized every month Spring-Autumn in Zurich (pop. ~700k city, ~1.5M agglo) and I'd estimate that there is a bit below 1000 bikes there ~each time.
