Selling my mac mini which has Lion installed.. Can I transfer the Lion purchase to the buyer of mac?
There are two types of Lion licenses.


licenses tied to one mac (hardware linked license)
licenses tied to one Apple ID (person linked license)


The former transfers, the latter does not. 

You can of course hold option when clicking Purchased in the App Store to download another copy of your Lion installer (or download Lion again onto your mac using recovery). The installer would not have asked for your Apple ID if it determined that mac deserved lion due to a hardware linked lion. You could of course install lion the first time on your soon to be sold mac, but the person buying the mac would be "without a paddle" if you hadn't prepared them that they might need to pay to reinstall their OS if their mac ever crashes or gets a new hard drive.