Two possibilities (in a nutshell):
a) if it originally came with 10.7.5, then the seller has to transfer the OS license to you by calling Apple (and there are no disks for Lion).
b) If the seller purchased Lion at the app store, then the license is forever tied to his Apple ID and is not transferable. He is obliged to erase the disk and reinstall the original system (and give you the install disks).
Since you won't be able to reinstall at this point because your Apple ID and the machine ID do not match (and the app store will detect that), you will need to either contact the seller or Apple (give them your serial number) and get above questions answered. If th answer is b), ask Apple to send you replacement disks for a nominal charge. With those, you can boot from it, erase your drive, reinstall the original system and then purchase Lion under your Apple ID.