FLJeepGuy

I do a bit of flipping, but I'm really picky on the bikes I work with. For me, I walk away from a bike with anything beyond a minor bit of surface rust on exposed metal parts. A rusted chain is no big deal unless the gears and derailleurs are rusted as well as I put chains in the same category with other wear items (brake pads, cables, tires, etc.). Just be aware, if a chain is badly rusted and/or worn, there is a fairly good probability that the gears may be well worn and need replacing too. Ultimately, it comes down to whether the parts and work required will still leave any profit on a flip and that really depends on your local market.