I was a personal friend (and customer) of Bill Schaefer's back in the 70's and 80's. I sometimes purchase and restore his mounts, when I can find them. Yours looks like a 9" mount built on the "large casting" set. You could verify that by measuring the diameter of the RA gear. The mount you have has the anodized RA gear, which is almost certainly a Byers gear. Later mounts had the maker's surname "SCHAEFER" integrated into the Declination shaft casting. I can't see that from your photo angle. Your mount appears to use the older Hurst "CA" model motor on the RA axis. Some of these mounts had dual motors - one on each end of the worm shaft. Again, I can't tell from your photo if this is the single or dual motor version. The Dec motor cover is missing. That's not good as it can allow dirt and metal filings to get into the motor. If you have a working drive corrector, that's a good thing. Your drive corrector appears to be the dual axis version, which adds value. Bill had drive correctors built by a number of different individuals. The motors and drive correctors are VERY reliable. I have several Schaefer mounts here with date stamped Hurst motors from the early 1980's that still work flawlessly. When picking up an older Schaefer mount, I usually find that internally, the ancient grease has been transformed into a sticky glue. I usually do a full mechanical teardown on them, which includes cleaning - and sometimes replacing - the RA and/or Dec shaft bearings. Time consuming, but not a big deal.
Schaefer mounts are fairly rare on the market. They are definitely "old school" but extremely stable, reliable mounts capable of carrying very heavy payloads. I currently have a very heavy (125 pound) 17.5" Newtonian mounted on a 9" Schaefer similar to yours. A modern commercial mount capable of carrying such a heavy payload would cost upwards of $10,000.
BTW, the other poster who listed the price range between $1000 and $2500 is pretty close to the mark. Judging by the limited info I have, I'd say the value of yours falls about midway in that price range. The mount should probably be serviced as described above. Feel free to contact me if you have further questions. Good luck with selling your mount.