With Gap, I wouldn't buy online (either from ebay or gap.com) unless you can and are willing to return them; their variance is just too high.
My previous pair -- bought over a year ago -- bagged out after every wash (usually went from "obscene" to "just perfect" in about 3 hours) AND also bagged out over time (9 months ago, they fit slimly but are now halfway to being boyfriends. And I've gained weight).
My current pair -- purchased over Memorial Day -- do NOT bag. I pulled them out of the wash, threw them on at their "obscene" tightness, and went to work thinking everything would be fine by the time I was done with my morning coffee. HUGE MISTAKE; I felt so unfab all day long :(. The label reads the same (ie, material content), so I don't know what happened. Like the other ladies here, I wash on cold, inside out, and never put them in the dryer.
My advice is to take them home, and wear them around the house for a few hours to see how they perform. To my eye, the dark wash is casually dressy ... and 50% off is a pretty good deal!