Trainers I suppose might be different, you can wear the soles out on those, though I have sold a couple of single used trainers before. I actually have a pair of practically new Etnies that the sole has started to peel from one. I have a pair of AlpineStars boots that had a buckle ripped off which would be a hell of a lot cheaper to find a secondhand single than replace both. Also a pair of Shimano shoes from mid 90's that are still in good condition but one cleat attachment has broke, haven't worn them since, all I would buy a single to match for, not that I actually look, too lazy and have other pairs.
Just looking through some listings I did find people splitting complete pairs up and selling them individually, now that's raising the odds
Just looking through some listings I did find people splitting complete pairs up and selling them individually, now that's raising the odds