@Risque: Sometimes the elastic wears out. It's rare to see 2nd hand pieces with the elastic completely worn. I see it more with the dresses that have elastic in the straps and they are hung by the straps for a long time. But it's a relatively simple repair.

Though at my size, I stretch the elastic out a lot and it's really sad. The armholes on some of my dresses (one-pieces, they're called) are kinda stretched. So if I want to resell, I'll probably have to go in with a seam ripper and replace the elastic.