Some tips;
-Always offer above what you want. 9/10 times, your buyer will haggle.
-Never lose tc. Even if your item goes under what you bought it for on the market, keep a hold on it until market prices go up. If you haven't noticed, the price of FPT has been fluctuating recently.
-LOWBALL! You never know if your seller is experienced or not. If he isn't, you've scored a gigantic deal.
-Stay away from hairs. Unless these items are being sold for a very small price, it's very unlikely you will make profit.