I would never, ever assign my LO contract to a TB for a fee!
You’re dealing with civilians. If they buyer stops paying, the seller has no clue how to handle the problem. If they buyer makes the payment but the seller does not pay the mortgage payments – then buyer gets a foreclosure notice, they have no clue how to deal with the problem. If this happens, Either of both could come after you with a lawsuit for putting them in a bad situation.
To protect both the seller and the buyer, (and you) it is better if you stay in the middle of the deal.