As seen in the imgur album here http://imgur.com/a/EKK44, improved trade price negotiation reduces prices for items across the board, but that devalues your selling price as well.
So you want to make two trades. You want to sell items at high prices with your worst negotiator, and then buy cheap with your best ones in two trades.
TL:DR Sell high with crappy trade price negotiators, buy low good trade price negotiators