What could be wrong internally with such lenses? And how easy would it be for a good third-party shop to fix? Some of these lenses are ridiculous bargains (I wonder why :P), and I can't help but be tempted to try.

Common things that break with lenses:

• a belt or loop is used to couple focus elements, the belt or loop has slipped off and thus no longer couples, resulting in loss of AF and/or loss of MF. This can be corrected by replacing the belt or re-engaging the belt onto the respective places.

• a lens element has shifted sufficiently to prevent proper focusing. Determine the element that has shifted out of focus and re-set it. Having a manual or a VERY adventurous spirit helps.

• a component has become jammed. a tiny screw, a bit of broken metal, someone dropping something into a gap between the rear lens element and the lens body... dissassemble and remove the problem item, reassemble.

• focus components sticking... grease/oil/etc. requires cleaning... after a complete strip down of the lens.

There are quite a few causes for lens AF/MF failure. Sometimes, it's something simple and can be easily fixed, netting you a cheap and usable lens. Other times, it is something that looks easy, but turns out to be pretty severe... in which case, you've spent some monies on a nice paperweight.

So long as you view the money you spend on the "as-is" lens as disposable and not an investment, sure, go for it. You can even find maintenance manuals for lenses if you search for them.