How many times do you think a gun should be field stripped by every looky loo to walk into the shop........and it remain salable?
Buy the gun, field strip it before you leave. Or ask the clerk or owner to do it for you. No gun store will balk at that.
Put yourself in the shoes of the shop owner....would you want customers dissassembling guns THAT THEY DONT OWN and causing damage?
The LGS nearest me, which does hundreds of sales a month and has 10 times that in foot traffic, never has an issue with me doing a field strip on a firearm, used or otherwise. Same thing with my local home run FFL dealer. That said, I never usually do it unless I am serious about buying, but they never make me hand over money beforehand. I've seen new guns with off centered sites and finish blemishes that I'd rather not have, so I always check these days.
Simple fact is most people don't do a field strip, most I've seen don't know how, so allowing the small percentage of people that want to do it to do it isn't going to cause some sort of massive wear on the gun.