Which OS version is on the Macbook and is that the original OS?
And, did the seller give you install disks?
This is important as any OS or app obtained at the app store is tied to the purchaser's Apple ID forever. The seller should have restored the machine back to the original OS, but you should be able to fix that
Post back with the infor I asked for.
FWIW, you are not running iOS on your Macbook.
If it's a mid 2010, then it would have come with Snow Leopard originally. The seller was required to wipe the drive and reinstall the original system and give you the install disks. If you want, you can contact him and tell him that; the licensing agreement states (in part):
Para 3B is applicable.
In any case, if he does not respond or doesn't have the install disks, you will need to get them from Apple - call them, give your serial number, and ask for replacement disks. They will send them for a nominal charge. The seller may have great ratings, but he did not do what he should have done. He should not have sold the Mac with Mavericks installed.
After you get them, insert the disk, boot up from it by holding the C key while restarting or rebooting, go past the language selection, then go to Disk Utility in the menu bar, choose to wipe the drive completely, and then reinstall. After that, you can update Snow leopard to 10.6.8, and then download Mavericks using your own Apple ID and you'll be good to go.
Although it's a pain, you need to do this now as you would never be able to reinstall the OS if needed, nor would you ever be able to update apps as they are all tied to the seller's Apple ID.