The problem with that is you have to keep the list up to date, then you're going to waste more time calling idiots who say "I am down to buy XXXX at any time. Just call me." You give them a call and get hit with "oh, man...that's awesome....BUT....I just had these car repairs and blah, blah, blah. Can you maybe buy the gun and keep it for a month in the back room? I'll be in to get it then, honest Injun."I think small shops could do well by keeping a list to match used guns with people looking for those guns.
I'm not talking about used M&P40c but stuff that is hard to come by in this state. For example if a guy wants to trade in his Desert Eagle. The shop could refer to that list and then make a few calls, perhaps selling it or taking a pass because they don't have an interested buyer at the time.
Most people want instant gratification when it comes to "wants" versus "needs". If they can't find X when they're looking for it, they'll turn around and get Y or so ammo or a fancy new handguard or something along those lines. They aren't good at saying "ok, this is my X money. I'm going to put it in the drawer and not touch it until X comes along."