I need to have put a product through its paces at some point before purchasing from any source, whether face-to-face or online. I've demoed stuff at GC and then beat them up for price matching (actually, they've always been very good about it) before committing to purchase from them. They expect it, so that's a pretty ordinary course of action when you're after something reasonably common.
When I want to demo something specific or unique, I've used a little more finesse. Recently I became intrigued with the Retronix basses by J. Backlund Designs, but no way was I dropping any amount of cash sight unseen on an oddball bass with a five-position selector switch and three pickups that looked it was designed for George Jetson. That's some next-level weirdness and is exactly why I wanted to demo one to begin with. So I found one in a locally owned shop and tried it there without making immediate purchase, and I simply told the owner up front that I was mostly interested in knowing what the instrument was capable of but probably would reserve judgment on purchasing until a later time. That put it on him to either accept or veto my request, and of course he was more than happy to let me play the one he had on the wall behind the checkout counter. I made sure there was nothing around me when handling his instrument that I might bang it into, I was respectful with amp volume, and I kept my most aggressive playing techniques to a bare minimum - common courtesy stuff - and I kept him engaged for information the whole time. Full-on demo plus inspection and dialogue ran all of five minutes, not exactly a massive time commitment on either of our parts, and although I didn't immediately purchase he knew very well that if and when I made the decision, he'd be the one who got my business. I learned as much as I needed to know about the bass in those 5 minutes.
Honesty and courtesy from customer + good business sense from retailer = winning combination. I personally would never buy anything - unique or otherwise - that I hadn't been permitted to demo, and I think it's a pretty rare retailer who doesn't understand that concept.
