Trying to re-format via internet recovery, no entry sign
The fastest method around a sticky recovery partition is to just boot your Mac in target disk mode and connect it to another Mac and have that OS run Disk Utility to erase / zero the drive.

At that point, you should have far less issues reinstalling the OS in recovery mode. Depending on the agreement you had with the buyer, you could just send the Mac to them with a wiped drive and let them sort out the reinstall since then their Apple ID will be associated with the upgrade / app store instead of yours.

To diagnose why your Mac isn't starting will likely require access to the install logs and/or system logs.


Can you boot the Mac into single user mode and get a usable prompt?
Can you boot holding the option key and then select your Recovery HD to get Disk Utility to run and zero the main partition/volume?