Per actual experiences. I've sold gear locally so in that case the buyers have been pretty transparent in asking for advice. In buying gear locally sellers are more apt to give advice vs take, somewhat ironic when I hear their system and its not up to par. If they're extremely prideful of system I'll try to hide my displeasure, if they seem open to an opinion or questioning I'll offer my two cents. I always try to pose my advice as suggestion, never want to to come across as condescending or superior.
The most curious encounters have been extremely expensive components, equipment hoarder, tons of equipment laying around unused or in second systems, and the main system sound quality is relative crap to the expense. In these cases always amazes me that some people don't believe in cables or rooms or AC quality, always something left out that leaves much untapped potential. In these cases I've made it clear they should at least consider the things they've left unattended.