When he talked to other characters about it, 'everyone' seemed to know that on certain items, gangers will only pay a certain percentage. The only way to 'learn' that seems to be through experience / trial and error. That is great and all, but it isn't very realistic.
Generally when people do business with each other, they know how much they are going to make. That is how they decide whether or not they want to participate in the transaction.
I see two ways to address this with code. One, have a second level option that allows a player to ask an NPC how much they will pay for something. For example, the character asks, "Do you need anything?" and the NPC provides a list. Then the character could ask, "How much for ?" or something similar, "What is worth to you?" maybe. The other option is to adapt the container (table / shelf / etc) code. Currently when a character buys an item, it asks them if they really want to spend the chyen. The same thing could go for the NPC. "NPC is willing to give you $$$ for . Do you accept?"