I also bought my 602's used but from a reputable dealer. I have a good relationship with them and I expect that if anything breaks, I can talk to them to try and resolve my problem. I've always found good audio dealers are willing to go out of their way to build relationships with potential customers.
I've only had one speaker failure(Infinity Reference) and it was because I abused them. I however didn't have problems talking to the local distributor(I think it was Audio Imports) and getting them sorted out, despite having bought them from an 'unknown' dealer.
I say, go for it...you'd have been very unlucky if they break and your chances are good that you can have them sorted out.
Sharp