It's very much caveat emptor.
However the minimum post counts serve to eliminate the one-hit wonders "I found this old nickel with a 1913 date on it in Grannys attic after she passed, will somebody give me $20 for it? Cause it's real old"
It's also reputation based - screw over somebody and they will publicly out you.
It actually works quite well across most of the boards (not just here) - since people are really members and posting to learn, share info, and expand their collections.
-----Burton
50+ year / Life / Emeritus
ANA member (joined 12/1/1973)
Life member: Numismatics International, CONECA
Member: TNA, FtWCC, NETCC, EveryCountry (online) coin club
Owned by three cats and a wife of 40+ years (joined 1983)
Author: 3rd Edition of the Sample Slabs book,
https://www.sampleslabs.info/