Which lens mounts can you use?

One would naturally think that lenses made for the same brand of film camera as your DSLR would be fully compatible, but that's not always true. Manual focus mounts were abandoned by most manufacturers when auto-focus was introduced -- such lenses will not even mount on Canon and Konica/Minolta/Sony. Even if the lens physically mounts, the camera interface may have evolved, creating various incompatibilities (e.g., the Nikon D60 cannot focus body-driven auto-focus lenses). In rare combinations, physically mounting some slightly incompatible lenses can cause damage. Check the WWW and the manual that came with your DSLR to see what's really compatible.

Adapters of various kinds are available to allow alien lenses to mount on your DSLR -- search on the WWW. Avoid lenses for which you find any adapters that contain glass elements.  The "universal" M42 screw thread, used on many manual-focus Pentax and Praktica cameras, is one of the most common mounts for older lenses; it can be glasslessly adapted to many DSLRs (but not Nikons) and still be able to focus to infinity. Adapted lenses of any kind generally require you to manually focus, which is slow, and it is easy to be slightly off and have things look sharp in the finder. Consider using a "chipped" or "focus confirm" adapter so that your DSLR's auto-focus system can tell you when the image is truly in focus. Chipped adapters may also enable or improve other features of your DSLR, such as in-body shake compensation, by telling the body the lens data that it was programed with (which you want to roughly match your lens). Most such adapters cost $20 to $30 and are widely available on ebay.

Note that just because a lens is sold with a camera does not mean it fits that camera. Postings often either do not identify the type of lens mount or identify it incorrectly. Similarly, it matters what focal length and aperture a chipped adapter identifies itself as, but very few sellers state what parameters they use. I had heard 50mm f/1.4 was the default, but the first adapter I got said it was a 105mm f/1.7.

Which lenses should you buy?

If the posting does not explicitly state the condition of a lens you're thinking of buying, ask.  Most important is clean, clear, glass. Avoid optics with fungus, mold, haze, and similar problems; a little dust is less of an issue. Radioactive yellowing of lenses, especially the very popular M42 50mm f/1.4 Takumars, is a serious defect that usually can be corrected by simply exposing the glass to UV light for a period of days to weeks. Glass condition problems are rarely visible in moderate-resolution photos captured by the lens; the seller should take the lens off the camera and carefully look at and through it to determine condition.  The aperture control also needs to work. Build quality of many old manual focus lenses is amazingly good, so serious mechanical defects are less common.

Lens technology has improved over time.  There are exceptions, but the older the lens, the less likely a wide-angle or zoom lens will do as well as a more modern lens.  Normal or telephoto prime (fixed focal length) lenses use simpler optical formulas, so old ones often are quite good. Look on the WWW for reviews of specific lenses and be aware that informal reviews often have a positive bias. If you're buying the lens to make images, you shouldn't be paying more for an old lens when a new(er) one with better image quality and/or features is widely available.

Don't forget that smaller-than-full-frame DSLR sensors have a crop factor which changes the angle of view, but not properties like aperture and depth of focus.     For example, if you're used to film SLRs, a 28mm f/3.5 wide-angle mounted on an Olympus 4/3 DSLR behaves like a 56mm f/3.5 with way too much depth of focus. On the positive side, crop factors have the "sweet spot advantage" of using the central portion of the image where quality is best. Crops also help extend the reach of telephoto lenses.