If you requested a trade just look for your trade in the black market, rate him negative and leave a comment on that.
If you didn't do that, you're at fault here, since rules states that you should always request a trade.
Advice for later:
Avoid buying anything from people who's low on TBM, got negatives and are registered recently. Usualy people with high TBM and zero negatives work hard to get/not to get them and tries to make both sides happy. Also don't believe in 2x cheaper prices, since they rarely exist
