Transfering Lion preinstalled license to new owner
Nothing needs to be done since the Mac's serial number is used to re-download Lion so no Apple ID is tied to that installation package for the OS.

This differs from purchasing a Lion upgrade from the App Store - that isn't intended to be sold, so you'd have to adjust the sale price or gift them a card if the deal was for them to get other software included with the sale.

I've seen people trip up over iLife since it does get tied to an Apple ID rather than tied to the CPU when selling a used Mac, so caveat venditor and caveat emptor is in order.