If old Singers are not common in your area, maybe it's not too much. They are fairly common here. Also, I live in a large metropolitan area with somewhere around 3 million people within 50 miles. So we have lots of opportunities. I would still offer less and see if they would take it.
Double check the wiring and see if it is cracked or "melted" looking. Machines with worn decals bring less than those with nice decals. Also, sew on it. Take your own fabric and thread when you go to look at it.
The first machine I bought was set up and had the light on when I got there. Under the presser foot was fabric with a nice line of stitching on it. It looked as if it was sewn on that machine. It wasn't. The feed dogs were totally worn down, so it couldn't have been sewn on that machine. It wasn't an expensive fix, thank heavens.
bkay