I was thinking of maybe an apple snail or two, but I've never heard of snails having disease that transmits to fish.
That really depends on the LFS, and their tanks. A single drop of water can introduce parasites/pathogens/viruses, etc. I'm personally not big on gambling with my livestock, some that I have invested many years in, so at the very least I would for sure qt anything going into their tanks for at least a few weeks. Personally, I wouldn't even risk adding snails from a LFS, but that's just me.Some carry parasites, but there isn't an easy way of detecting them. If your fish and tanks are healthy, you should be okay most cases