Make sure components are easily replaced rather than proprietary. As things wear/are worn, you will want to replace so make sure compatible parts are readily available. If you want to upgrade to discs, make sure the bike will accommodate. Check for obvious things like cracks, rust, etc. I always figure if the frame is good the rest is eventually getting replaced anyway. Just take into account cost of replacements for components and new wheels or whatever when you are bidding. You don't want to buy used and spend more than a new version to bring it up to your desired level (well, maybe you do, if it's a classic or you just love to tinker). I've probably put more into my Bike Friday, bought used, than I would have paid for a new one but the cost was spread out over time which was more manageable for me and it gave me the time to figure out what I wanted to change. I would not have known to order it the way I ended up configuring it. And there's some emotional satisfaction/attachment to bring an older one back to life.