How do I buy a used lens?

[BigSteveG]

I have arranged to purchase a 21mm Biogon and will be picking it up on Tuesday. I don't have the luxury of borrowing for a test run and I am expected to pay cash. Does anyone have any advice to offer on how to inspect the lens? Obviously I will take a G body along to make sure it focuses. But beyond that I guess I'll just look for any physical problems. All suggestions are welcome!!

[FrankS]

Take a small flashlight with you. The more powerful the better. Shine it through the lens to detect any haze. This isn't really applicable to a lens as recently built as a Contax G lens, but may be useful.

[peter_n]

Also request an "inspection period" of say a week with a return/cash back guarantee. Long enough to shoot a test roll with the lens to make sure it is OK. If you are refused strongly consider not buying the lens. There is always another one somewhere...

[John]

Do as FrankS has recommended. Shine the flashlight through in both directions. You might see some fine scratches, sometimes called cleaning marks. Fine ones are likely just scratching the coating. A newer lens like the 21 is hardcoated and potentially mark free.
Remember you are not likely to see any marks unless you shine the flashlight in from the back.