I think it should be as in test driven cars, regardless if it will end up the same within a few days/gigs etc.
It should not be sold as new, but rather demo model with a discount
Only exception with a guitar to this I see, would be if you say you want X model, and the shop says, I only have the one on the wall in stock mate, take it or leave it at retail mate. Then its up to you
But if they've new stock they could offer you, or didn't inform you it was a demo and you agreed to pay the full price regardless, then thats not on.