what do i like? good price, consistently available, good product, professional, good sense of discretion, on time for a deal...
what do i not like? rip off prices, fake product, cats acting like fools, doing paranoid shit during the meet that just looks _more_ sketchy etc.
usually for me, the only things i buy from someone "cold" is bud and dope. bud is my DOC so i'm always looking for more good bud guys to add to my phone book. i usually start out small buying a quarter, than a half, then if they do OK with that they go into my regular rotation at an ounce a month. once we've been doing business for a while, often i'll offhandedly ask if they can get rolls, trips, pharmies, whatever i'm after... but generally i won't buy that stuff until i've known the guy for a while and there's some trust there. dope, different story, the potential to get ripped off is just part of the game... but at least you have some assurance that whatever powder you're buying, if it's not dope, at least it's not some unknown RC that's gonna make you wig out too much.
but really i haven't bought trips from anyone but a mail-order supplier in almost a decade. you just don't see that kind of stuff around here so much on the streets (anymore?) if i were to make an exception it would only really be in the festival environment, from experience, most of that stuff is legit, just try to read the guy as best you can. i feel better if i'm trading drug for drug in these sorts of deals for some reason, rather than giving an unknown guy cash.