This sort of hits home. When i was in highschool, some classmates of mine went into the local 7/11 with water-guns and "robbed" the store. They were actually playing around, had no intention of robbing the store, and their guns were the clear plastic type, full of water, with an orange muzzle. Sort of hard to mistake for the real thing. Unfortunately, the police showed up at their houses later and arrested them. Apparently the clerk was not amused and neither were the police.
Still, as to the original poster. No. I would not shoot the man if I knew for a fact that it was a toy gun. Now, that depends on what you mean by "toy." There are some BB guns that can do serious damage, possibly blind, or even kill somebody if placement is just right. So if it is a BB gun, then I might be tempted to shoot if I thought I was in danger of being shot with something like that. If it were an airsoft gun or a cap gun or something like that, no. I would most likely use some of the moves I learned in Aikido class to simply take the gun away from him. If he had a backup weapon, like a knife or something, I'd draw my gun at that point.