Originally Posted by Prism99

Cosmetic condition is a big factor in pricing old machines. A machine head and cabinet in better condition will fetch a lot more money than an abused one. Where I am a standard condition treadle machine and cabinet (very well-used looking machine with scratch marks on the bed but a clean machine head and decent looking cabinet) will sell for $50 to $100. One in really good condition might be $150. One with a really beautiful cabinet with both machine and cabinet in top condition might be $200. (Other parts of the country may be more expensive; not sure.)
Thank you for your expert advice. I do appreciate it so much.

Treadle machines are fairly difficult to sell; not many buyers, and they are heavy to transport.

It's fairly easy and inexpensive to replace the leather belt. However, you want to make sure the machine actually sews a stitch. That's why it's a good idea to bring your own fabric and thread to test out the machine. This also ensures that there is a bobbin case in the machine. (Bobbin cases can be replaced, but they usually run about $30 plus you can't be sure the machine actually forms a stitch unless you have the bobbin case.) If there is no leather belt, you can hand rotate the wheel to make sure stitches form. This basically ensures that you do not have to have a technician go over the machine and fix problems (an added expense). If it can form stitches, you can clean and fix up everything yourself.