I wasn't thinking of anything as complicated as a feedback system. If the Yahoo! search system was working as it should (and it is not) then a simple search by username should give you a list of items that a person has OFFERED / TAKEN / WANTED and that is at least a start to making an assessment as to who is genuine and who might not be.
If the system was extended to require requests to be logged as well then you would in addition be able to favour someone who has been trying to get hold of something for a while or alternatively disfavour someone who request anything of possible value.
A web interface might make it easier but it could be done within the existing Yahoo! framework if they could get their act together and make their search work.
Andrew