check the buyer/seller thread stickied at the top of the forum, and use your head, you will never be %100 sure that you won't get ripped off, but the majority of people here are great about sending stuff and keeping on top of things in transactions....
i haven't bought anything yet but i've sold a few things. I try to communicate a lot with the person who i'm selling stuff too and i use paypal so they're can be extra confident. but you should always provide your buyer with a tracking number and make sure they message you after they've received the stuff so that you know it got there ok and that they're happy with it.
just treat the buyers and sellers how you'd want to be treated.....and let's hook each other up and give each other some good deals ya know.
Yeah, use paypal, get some contact info, like a phone number if it makes you feel better, but check the NSTOR rating system (look in the later pages of the good buyer/seller thread stickied at the top of this forum)...generally though people on here are good about dealings and such...but use your head too, someone with 1 post and selling a bunch of gear super cheap is kinda fishy, could be they are lying and don't have it, could be that it's stolen, etc. Oh, and ask for pics of what you're buying... But seriously, I've had good experiences here...and I've bought and sold a lot of gear, I'd say at least 50 transactions.
"Nothing is impossible. You are only limited by fear and even that you can overcome."