First you ask at DA if you are allowed to sell your licenses.
After that, you post on the forums that you are selling your licens for a certain price. Do not mention any id's yet.
When somebody contacts you and you reach an agreement on price, you give that person the license id (only that, nothing else) per pm or email so he can check and verify with DA that this is a valid license and is allowed to be sold.
After verification, the payment is done and you can ask DA staff to transfer the license to the buyer.