How to check and how much to pay for used slicks?

I just found a few pairs of used slicks locally to me.
What do I look for when checking them?
How much is a good price for how much use left in them?
Example
10 runs left=$50.00 each???
20 runs left=$100.00 each???

Do slicks have a date code on them?

Ditto...check for date code, wear.
Guess guys use others slicks, Id be hesitant to though. Why are they selling them. :beers:

The tread face will have depth holes in them. Check that I would expect at least an 1/8 inch. And look at the sidewalls for cracking. Anything with 10 runs left, 20 runs left is junk already.