What to look for when buying a used lens?

I am about to purchase my first "L" lens, and it's used from Craigslist. Before I spend a decent amount of my hard-earned coin, what do I look for?

I will request the date-code, see if all the paperwork is there, along with the box, and obviously test it on my 50D. I did see it's owned by a professional photographer currently.

I would appreciate further insight.

Dust - because it usually hurts resale value
scratches on lens - because it usually hurts resale value
damage on lens body - because it usually hurts resale value and shows signs of abuse
Box and warranty card - because it usually HELPS resale value
Accessories - ie lens hood, carrying case, tripod ring
Additional accessories like protective UV filter - helps with resale value
Sample shots - verify no internal problems