Wouldn't the infringement be on the original seller? They sold you the copy of the software, and you bought it in good faith that they have removed it from their computer.
Now I haven't read the EULA (is it called that on Macs??), but a majority of them allow it to be installed on only one computer at a time. This means if you buy a new computer, as long as you remove it from your old one, you're covered when installing it on the new one. Sometimes you get lucky, and it will say you can use it on one computer at a time. This will let you install it on any computer you want (think desktop, laptop, work, den) and you're fine as long as you use it only on one at a time.
Either way, if anything were to happen, you're the one in possession of the software, they would have to explain how they got it installed on their computer without a disk.
