I have a Cynthia Rowley on consignment right now. I took the bag in, they mark a price on it after you tell them how much you paid. If the bag sells, they take a % of the sales price. If it does not, you can re-consign it or take it back. I think the consignment period for mine is 60 days.
I've sold a few things through consignment stores. Their policies vary. Some will buy your things outright, and then sell them at whatever price they choose. Others (most, I think), put your items on sale, and then give you 50% of the proceeds. Note that a lot of stores will lower prices as time goes by, too, so you may not get a guaranteed minimum for an item.