I just bought a used EF-S 17-55mm f/2.8 IS USM lens.
I got a really good deal because I was able to bid down the price as there was dust in the front barrel and because the lens exhibited noticeable zoom creep.
Too late now, but generally when it comes to used, if something ain't right and it's a private sale (i.e. no warranty) then walk away. You might think you got a bargain, then vendor might think they got some decent bucks back for a paper-weight.
Unless you've repaired lenses before and have the ability to recalibrate you risk making a smallish problem into a biggish problem..
Use some of the buck you saved to let a professional look at it... or you could be chucking what you've already spent away.