830's are great machines! We've sold used one for $300 to $500, depending on the shape they're in, and what accessories they have. As was mentioned, the camstack gear (about $33) and vertical shaft gears (about $73) are important to check before you sell. Also, the foot controls can be a problem. If they sputter when you start, they need a capacitor replaced -- that's an easy fix. If they're older, and have problems, the whole thing may need to be replaced (about $34). The stitch selector lever (on top), and feed dog knob (bottom, right) tend to gum-up if they sit un-used for a long time.
Good Luck!