Cheap used bike decision chain:
1. Huffy or other "bike shaped object": DO NOT BUY
2. Does the seat post move? No? DO NOT BUY
3. Spray painted over components? Only buy if you expect to replace them all, indicates poor care (or stolen) generally DO NOT BUY
4. Missing major components (brakes, shifters): DO NOT BUY
5. Major, major frame rust: only buy if you are looking for a project, can be fixed.
6. Rusty chain: -$15.00 off price. Needs a new one. Indicator of rest of bike care/wear.
7. Heavily rusted rear sprocket, wear: -$15.00 off price
8. Lightly rusted rear sprocket: easy to clean, don't worry about it.
Always assume new tires and tubes, maybe tire liners (approx $100, plus or minus, depending on what tires you replace them with)