When buying a used saw, examine it carefully. More than likely it will have some rust. Also, it will probably need to be sharpened and have the set adjusted correctly. All of these things can be done without too much trouble; the only real deal-breaker is a broken tooth.
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There are several types of tooth designs on these saws, but the most common are champion (or tuttle) and lance. The lance tooth is best for softwood and the champion best for hardwood, but they are always interchangeable in a pinch. If you find a saw with a chipped tooth point, don't worry, it'll eventually be sharpened away.
Illustration courtesy MOTHER EARTH NEW editors
The designs for a DIY sawbuck.
Illustration MOTHER EARTH NEWS editors