How about this for the good seller/buyer problem:
What if we had another tab in everyone's profile that was "Buyer/Seller" or "Reliability" or something like that. You could go in and rate people or leave comments about a transaction. To prevent people from fucking with other people's rating, you could have a feature where once a transaction is agreed upon, the two members involved get one rating of each other.
Example:
Newschooler A posts goggle for sale, Newschooler B wants that shit.
Once A gets the money from B or is sure the sale is going down, he could send a "Reliability Rating Request" to B. (Lame name, but you get the idea.)
B approves, they each get to rate each other once based on whether they were a good buyer/seller.
If A took a while to send the goggles, B could rate him 2 out of 5 or something. If they came and B was super stoked on them, he could change his rating for that sale to a 4 out of 5 and maybe have a section for B to comment on the transaction process. A could also rate B.
Next time someone is thinking about buying from A, they could check in his profile about his past transactions.
I dunno, it seems like it could be pretty helpful and a little more tamper-proof than just a straight up rating or something. Thoughts?