I've not ordered cards that wasn't already in the store, but usually they have a nice selection of a little of everything. It depends entirely on what you're looking for.
They have cards (for many different types of games) for sale. If they don't have what you're looking for, they can either order it or someone there can help you find it. If you want to sell cards, they do occasionally buys some.
Thanks! Your answer is awaiting moderation.