Is a rusty saw usually worth fixing up?

For a hand saw, surface rust is usually less concerning than a bent or kinked plate, badly damaged teeth, or a cracked handle. Ask for clear pictures from the side and check that the plate looks straight. If the price leaves room for a little work, light rust can often be cleaned without aggressive sanding.

Depends what kind of saw it is. With a bandsaw blade, look closely for cracks, damaged teeth, or a questionable weld before putting it on a machine. Clean off loose rust and protect the blade afterward, but don’t run a damaged one. Rust can also wear the guides, so don’t just install it as-is.

I’d judge the whole tool, not just the color of the rust. Pitting may remain even after cleaning, and a saw that is badly bent or missing lots of teeth may not be a useful buy. If you can’t inspect it in person, price the purchase as a gamble and ask for close-ups of the plate, teeth, and handle.