I'd start with the condition of the cooking grates. If not cared for they can get pretty rusty and add to the cost you wind up paying. Check overall for dents or dings or chipped porcelain. If the smoker is not stored with the middle section upside down, the bottom section can fill with water so check there for rust as well.
Make sure all parts are there. My '09 included a cover so if that's missing there's something else you might need or want to buy. It should have three grates, a charcoal ring, water bowl, bottom, mid and top sections.
The older ones have a round handle on the door and supposedly the door was one area of improvement as well as the addition of the thermometer in the lid. IMO a smoker with those features is worth more.
I don't see many used but occasionally they do come up and some times even at a terrific price.
Good luck!
Edit: There are 2 22.5 WSMs listed in Chicago Craigslist right now.