There are many threads about this. However, this is the short version:
1. Get your buyers username
2. Ask your buyer to create a security question and answer just for you (create a new one, don't let him use the one he already has on file. He can delete this 'temporary' question and answer after the transfer)
3. Call MyDomain.com support and give them the relevant information. They will push the domain to the other person's account for you.
It's a pain in the butt compared to other registrars, but that's what you gotta do these days.