What I do is that I log into the TP, and it says what items sell for what prices, and you can put your mouse over individual items to see what the NPC offers. If the TP price is better than the NPC by enough, I sell on TP. If not, I sell to NPC. If you sell on TP, also check how much the price difference is between highest buy price and lowest sell price. If the difference is large, you might as well sell for more than the lowest buy price. If the difference is tiny, you might as well sell at the lowest buy price and get your cash right away.