I used to have the same bike. The seats on an 06 FLST mount up like a deluxe. Post the ebay link and I might be able to help. I know I bought a passenger seat off an 07 heritage for mine back in the day and it worked.
Hmm hard to say from the pic with. You just want to make sure it will lock in place like the stock pad does at the main seat bolts. You might want to call a local dealer and ask them if they can be swapped.
bought the seat, it DID NOT fit... sucks. the two front bolt holes are closed on the FLSTC seat (FLST are open, easy-detach) and the holes on the FLSTC seat are about 3/4" closer together than the FLST... plus, the rear of the FLSTC seat has a female connection underneath, whereas the FLST seat has a fixed, male hand-tightening bolt affixed to the rear of the seat.