Check the Glue - is it clean? is it all there ? are they still sticky (warm skins should be somewhat difficult to pull apart)?
Are the tip and tail clips in tact? are the pent, cracked, or is the skin pulling apart where they connect?
Is the traction side of the skin still there? is the hair falling out?
Are they big enough to fit your ski?
.... lots of quality used products out there, if you can check them out yourself it should be easy to figure it out. I would never buy used skins sight unseen.
JoeStrummer-"No avy training but I've watched K2 so many times I think I know what to look for."