Came with lion installed.
We know it has Lion installed now; the question is:
Did this machine come with Lion preinstalled initially??? In other words, did the original owner purchase it with Lion pre-installed or did s/he purchase Lion from the app store?
That is extremely important because of the different license restrictions; if you don't know the answer to that, then please post the exact model number and identifier so we can look it up.
If the machine came with Lion preinstalled initially, then the seller was/is allowed to make a one-time transfer of the OS when selling the machine. However, there is no info from Apple as to how this is to be accomplished; so, if this is what we are looking at, it would be best to call Apple and find out how to get the OS transferred to your Apple ID. For everyone else's benefit, please post Apple's answer to that here.
If the machine came with Snow Leopard installed initially, then my previous posts apply. You can buy the USB drive; however, according to the licensing terms, you still need to have Snow Leopard installed first to be legal. Hence my suggestion to buy the retail SL disk and then purchase Lion from the app store.