When you pawn an item, you are taking out a loan using your valuable as collateral. The pawnbroker will agree to give you a certain amount of cash and hold your item until you have paid back the loan amount plus interest and fees. Should you choose not to repay your pawn loan, the pawnbroker keeps your valued item and is able to sell it to recoup the loan amount.
When you sell an item at a pawn shop, you simply bring in your item for sale. You and the pawnbroker come to an agreement on an amount that the pawnbroker pays to you. You leave with cash in hand, and you have no further obligation to the pawnbroker.