Make sure the seller has some credability... Search the forum for their username.... search Google for their username. Check their Heatware, search Google for their email address. If you have their real name, search Google for that too.
If a deal is $50 or more, I do all of that and more to make sure that the guy I am sending money to, is legit.
The easiest thing to do, is to search the forum for their other posts and see what they have sold before, then check their Heatware to ensure they actually had the transaction.
Heatware is generally good, but there have been cases of people creating multiple Heatware accounts and then just leaving feedback to each other account, to appear as if it is from multiple users.
Another thing, if you are scared enough, make them give you a phone number and call them up to ensure everything is ok. If the person is a good trader, they usually don't have a problem with that.