We have done both and used TTS for our resale purchase. It was easy and a great experience.
It takes between 6 - 8 weeks from start to finish, depending on ROFR. If Disney takes the contract, then it is longer as you have to start over as a buyer.
But, if you see a contract you like, contact the broker, and discuss it with them. You then put in an offer to the seller. Once agreed upon, goes to Disney for ROFR. Once that comes back, it closes.
If you get a contract with the same UY and title it the same way as your other contracts, it can be just added to your membership (make sure you tell them this).
If it is a different UY, or different owners, then it will be a new membership, with a new membership #. If this happens, then combining the points between the two memberships is a bit more difficult.
Good luck!