Needs to be more formulaic.....
Barter Guy: Hey, I've got this item worth half of the value of yours, let's trade!
Person: I don't know, I could probably get more for my item elsewhere.
Barter Guy: But you really want this, right? (says something relevant to why the person wants the item, list of phrases: It's for your kid(s) / It's for your wife / You need this for your job though right, think of how much money you'll make using it / You could sell it but then you'd also have to look for one to buy, I've got exactly what you're looking for (this is sort of like the Pawn Stars "cash money" phrase) / etc.
Person: Okay, I'll let you ream me!