Did you keep the old parts (fork & stem)? Do you have a photo of the original stem? It is possible it is repairable.
I don't recognize that model. The seatpost attachment sounds cool, but would be a problem if you ever decide you want a different post.
501 is a mid quality tubing.
As it is, I think you'd get around $200 for the bike.
You'll get a bit more with the original fork and stem, but I doubt a fortune.