but the agreement was reached as the potential buyer agreed to purchase it at a specific price and now he has backed out, so he has breached the agreement.
Well at the end of the day if you win a bid at auction or buy it now on ebay then you are obliged to pay, just the same as you are any other auction. However, iTrader is there as gage to help buyers and sellers establish anothers trading history. I think in this case you should be leaving negative rep and not negative iTrader, it may be a simple case of someone thinking they had the funds but did not etc... etc...
I think for a 1 sale offer, like an auction for a website or domain name once someone posts "sold" or "sold for BIN" they need to be held to that. Other potential buyers will move on and you'll actually be losing sales because of it. If it's something you're offering limitless copies of then it's a different story. You've gotten a free bump and they are helping your visibility. Certainly, they shouldn't be posting "sold" if they have no intention of buying it, but i don't think it's worth a negative itrader. Maybe contact them to find out their objections, or have a counter-offer. Posting negative itrader right away may cause you to lose valuable feedback or even a future customer.