Lion came out in July 2011, so if you bought it new in late 2011, it would have come with Lion.
The way you have it now, if the new buyer ever goes to reinstall Mavs he will be asked for your AppleID.
I think the best is to get you back to Lion, then the new buyer can login to the new machine a go to the App Store under their AppleID and "purchase" the free Mavs upgrade under their own name.
So, to get back to Lion do a command-option-r boot to Internet recovery. Once the recovery screen comes up start Disk Util and select the drive itself at the very top (above Macintosh HD) and erase the drive in Mac OS Extended.
This will erase the entire disk, including the Mavs version recovery partition.
Once the disk is erased, just quit Disk Util then click install OS and you will get Lion installed from Apple's servers. You will not need your AppleID, since this install is tied to the machine's serial number.