It varies for me. I bought a stacked wood Kopa with a hairline crack in the wood. You have to look close to see it, but it's there. At the time, the knife was discontinued and the 2nds price was super low, so that was a small flaw that I could easily overlook, as the crack blends in with the scale. I also don't mind slightly off center blades, small discolorations, dull blades, or small scratches. Those are all things that I can either fix, overlook, or will happen to the knife with use anyway.
Reasons I've passed on some of the seconds are: rough action; super sticky locks; major damage to the ornamentation of scales (such as big chunks missing out of the inlay on a Kiwi, missing sections of wood on the Sage 4, etc.); snapped off tips; seriously messed up edges (though I did get a Kahr Arms Delica that a third party sharpener had really messed up but it was crazy cheap); and cracked scales (like the glow in the dark Manix, where the crack was quite obvious).
For most of the reasons I have passed, I was able to find a second example with lesser flaws and, in the cases of quite a few of them, there were tons of that particular knife to pick and choose one that was closest to perfect. For knives that I can only find one of, or that are long discontinued and normally expensive, I may pick one up that I would normally pass on had it been a more common knife. For example, I got a flame anodized fluted Ti Military at a seconds sale and I can barely open it with my thumb because the action is so terrible. However, since they were long discontinued and it wasn't one I was planning on really carrying, I bought it anyway, even though on just about any other knife I would have passed on it for having that same flaw.
Another was a Stretch FRN that looked like someone had taken steel wool to the blade, as it was completely covered in scratches. Normally I would pass on a knife that ugly, but it was so cheap I got it anyway. I figure that the ugly blade gives me a good excuse to learn how to do a stonewash since, if I fail, it can't possibly look much uglier and if I end up ruining it then I'm out hardly any money.