I don't really see it as that big a deal.
I tend to leave mine up, mostly because of laziness. Most of my stuff is on the lower end though, so there isn't much worry about someone using the price as a bargaining tool. If my cheap card has sold for X and someone offers you X+$1 you're probably looking at a pretty good deal
Sometimes I'm mildly disappointed when I go to look at something and it's just listed as $sold, but only a little, as most of them are more than I can/will spend right now.
What I'd like to see is that "Sold" added to the title. That would actually be pretty useful whether the price was left up or not. (Not like I do that myself........yes, that's a thing I should work on. )
Steve B