There's usually a Bike Swap Meet in my town once or twice a year. The bike clubs/vendors pay a fee to have a couple of tables to put sale items on. Individuals can also sell items but may need to do so in the parking lot to avoid paying a fee.
No entrance fee to buyers/browsers.
I sold my beloved vintage 10-speed with a too-large frame at a negotiated price.
I swapped out the nice saddle before selling the bike, using a cheap saddle I bought at the previous swap meet.
I've also bought like-new bike shoes and have sold jerseys/jackets that I decided I didn't like.
Another bike club member got a $$$$ almost-new road bike for a great price. I believe the seller was known to the cycling community and his ownership of the bike and reason for selling was trusted.
The local bike stores also use these as an opportunity to clear out their bargain bins with deeply-discounted merchandise.
The last swap meet has been at a local brewery in a central location. It was a nice social event for lots of cyclists.
Previously the meet was held in conjunction with some bike races. I think it also brought some first-timers into crit racing.