Here is the manual for it:
A common issue with Slant-O-Matics is the knobs for selecting stitches get sticky. The secret to fixing it is sewing machine oil on the posts either side of the cam stack, a hair dryer, and some time. (You can use that to your advantage to negotiate a possible better price, since it is such a trivial fix.)
Avoid sewing machines on eBay, as they often arrive damaged due to poor packing. IF you can try it in person, you're much better off.