I sold a handful of items over a few months' time but that was without really actively working on it, promoting it, etc. It didn't take many sales to make me happy because that's when the site was still free -- although the fees now are incredibly reasonable.
You might check out their forums for tips and whatnot but, really, it's like anywhere else. The better your items, descriptions, photos, etc. the better you'll do.
I imagine that one could do really well there with the right kinds of items because it's extrememly active with lots of new stuff added every day.