make sure it works before you sell it,and be honest I'm not that dumb
I've been attending hamfests for 20+ years and I'd say 90% of the gear I've bought has been as advertised. I will also say that I've been rarely lied to when buying gear via Ebay or various forums... but I've been outright lied to my face on several occasions at hamfests.
When buying hamfest gear, I always have a good idea what the stuff is worth and always assume a piece of gear has some sort of problem. I make offers, generally low, based upon that assumption. I will not pay "top dollar" for a piece of gear solely based upon a seller's word that it works perfectly. If I want to make a substantial investment in a piece of gear, I'll be buying from a professional who backs the gear with a warranty, service, and support.