I have never been comfortable dealing in used sifrei Torah, only new ones, however recently a shul approached me with three sifrei torah they do not use which are in possul / questionable condition. They don't have the interest or the funds in fixing them and they have plenty of new kosher ones. They could use the money for other things in the shul if they sell them.
I've never looked into this and I know there are complicated halochos about a shul selling sifrei torah.
What is the din if:
1) The shul wants to sell their old / disused seforim to another shul who could use them, and this other shul has the funds and motivation to repair them and bring them back into use. But if they remained in the first shul they would likely not be repaired or used. There may be a middleman / socher involved in this process who will make a mark up for arranging the sale and organizing the repairs.
2) A shul wants to "trade in" a few old / disused seforim for a new one. The dealer will buy the old ones off the shul and repair them and resell them to other shuls who cannot afford a new sefer. The proceeds will go towards the new sefer the shul is buying, less a cut for the dealer.
I assume there are people on this forum who have experience in this.