Been to a few around this part of the country.
Given we mainly aim for vintage/retro one, they are of course older stuff.
There is a mix of people just selling old stuff, retail trades, and them that know and specialise in areas.
They seem to be mainly road based, well it been around far longer and is probably the bigger market at the moment. But MTB and old MTB are hidden in there quite often.
You will learn to try and identify things, keep an idea of what things sell for, haggle a bit and that youn don,'t need any of what you bought.
No idea of the selling side of thing, but I may do soon as I have gathered quite a bit of the no idea why I bought it, but was shiny or maybe I could repair it....