Apple actually places a lot of trust in its users. This will fall into an ethics thing more than a will it work on my system. There is nothing that stops someone from loading a piece of Apple software on their computer and then selling it to someone else, who loads it on their computer and both will work, both are upgradable but only one is legal. There is only a problem if both are opened on the same network.
So the question is who are you buying it from? Do you know them? Are you sure they have removed the software from their system? If you can be fairly certain, there should be no issues on your part.
One of the great things about Apple is their take on all this. They make it so easy for their real customers in getting their OS and software up and running without jump through hoops with internet registration/validation/dongles, we as customers need to make sure we are paying for legit software. If we police ourselves, there is less need to be policed by others.