Years ago, I got a bit hot under the collar about an offer, which on reflection I shouldn't have.
Not a musical instrument - a motorcycle. Me, vaguely attracted to some other bright shiny thing in the shop: "How much would you give me for this?" Them: "We're not really interested in that" Me: "How much?" Them: "£1000" At which I took umbrage (scrap value of said bike was about £1500), but on reflection I was really ignoring what they kept telling me, that they didn't want it. It did take me many, many months to get to that point of reflection though.
I think (going back an awfully long way to the OP) there is an awful lot to consider in terms of a "straight trade". The shop has bought in an item with a certain level of mark-up. There's VAT considerations on this as well. It becomes a lot more complex in terms of profit and loss than "you're selling A for £x, here's B that I'd like £x for". There's a few on here that have actually worked in shops who could give a more informed view than me.