Most shops have a consignor contract, I'd request it. If they don't have one, ask them to write one up, so there is no confusion. On your end, you'll want the store to create a list (or you create one) that outlines the items you give them. Have them sign and date the list so you have some sort of proof that you gave them these items. You should also let them know that if it doesn't sell in X number of days, you'll be replacing the item with something different, or removing the item. When the item sells, this is when the store will pay you the selling prices, less their percent for selling it.
Hope this helps,