i dont think it should be allowed to give a negative feedback.
the system we use on my srt forum allows you to give info on the transaction, and once you do it raises the persons "Trader Rating" one point, showing they made a transaction.
then you can click that number and it shows what was said about the transaction and a link to the thread where product was sold/bought.
the system also does not allow you to give someone feedback/rating over and over. i believe 2 times consecutivly is the max. after that you get a message somthing like "you must recieve feedback from someone else or give to someone else before giving more to this person"
its the best system ive found. take a gander.
srtforums.com
same user name there for me if you wish to look up my name, tho i havnt been on there since i parted my car out.