I wouldn't have bought anything from a tank that's actively infested with marine ich. It's one thing if it's a low level latent infection, but if it's a heavy infestation then I would just wait. It's presumable that the parasite could latch onto the snails shell and hitch hike in that way.
But yes, ich lives for a short time in the water column. I can't remember exactly how long, but it's something like 4 - 6 hours or so. So yes, you could have introduced ich to your tank.
As to being concerned, there's nothing you can do. If it got introduced then there's nothing that can be done about it at this point. You will have to wait for fish to show signs
Mebbid's Fishtopia: Various builds
"The simplest explanation for some phenomenon is more likely to be accurate than more complicated explanations." -Occam's razor