A problem might well be that A - it'd be a nuisance to set up guitars every time a possible buyer asks (because loads do, all the time, few with even a hint of true interest) and B - the overwhelming majority of players want light strings and they'd need to change it again straight away if you weren't to buy. Also, they might use a tech who isn't a direct employee and would need to pay his full fee. All the guitar shops I've worked in were in that situation, the techs got to use the space rent free as long as they did warranty repairs free, but they'd charge the shop for bits and bobs that weren't "needed".
All the problems come from the fact that obviously 99% of people who show interest in a guitar wont ever buy it. It'd be great if you could easily tell the guys that are serious, good customers from the arses that circle guitar shops asking staff to go to great lengths for stuff they'll never, ever buy. Or if they do, off the internet.