Fair price on used tires/rims?
 

I found a guy in my area (former owner of my car) who has used snow tires for it. They're 17" Blizzak WS-50's with about 5,000 miles on them. The rims are Kazera's, worth about $170 retail.

What's a fair offer on them?

Miles don't matter as much as how much tread depth is left, if the tires are scuffed, cupped or otherwise unevely worn. I've seen people ask anywhere from $600 to $800.