Checklist/tips for buying used tire
 

I'm going to be picking up a set of used winter tire and since I've never purchased used tire before, I'm just wondering what I should check.

I can only think of checking the tread right now.

Any suggestions would be great, although I'm quite limited in what I can test since I'm going to the seller's house.

even tire wear, nails/screws in the tire.

The age of the tire (using dot numbers - 6 years is usually the point you should be changing tires)
Weather cracking
Tire wear
Tire repairs

As for winter tires, after a certain point a winter tire no longer has the siping/cuts to BE a winter tire and thus becomes useless in the winter (aside from the softer compound). So watch out for that (look at a picture of a new winter tire and you'll see)

How do I check/look for siping/cuts ?

I google'd it up, but don't know how it looks in real life.

http://www.tirerack.com/images/tires..._dir_ci1_l.jpg

those zigzaggy lines are sipes.

if the winter tires are worn enough they dissapear/are not that deep.

I would check the inside of the tire to look for belt damage/cracks in the rubber, as well as the sidewall for anything that looks like it rubbed against a curb.

Most people seem to neglect checking the manufacture date on the tire. It's usually 4 digits circled (ex: 3409) where the "34" means the 34th week and the "09" is the year 2009. There are a lot of used tires for sale where they are 8, 10 years old but still have a ton of tread left but as a tire ages, the materials also deteriorate. Normally tires should be changed every 6-7yrs. It's also a good way of knowing whether an owner has possibly replaced 1 or more tires in a pair/set.

It'll look something like this:
http://www.pjrider.com/TireCodeNumbers.jpg