I mean first you need to transfer the name to a new level 4 account with no recoveries or JAG and then sell that account with the username (it's login, password and whatever other information that is required).
You cannot tell somebody when you're changing the username because anybody can snipe the name, if you sell the account with the username and leave it up to the buyer to transfer then they feel more secure knowing they're not going to get scammed and knowing that they control when the username will change.
tl;dr, you can't sell a username without giving them ownership of the account with the username since 95% chance they will lose the username.