If you do not send an offer within 15 minutes, or the buyer doesn't accept the offer, the listing will become locked and a dispute may be opened.
If the buyer doesn't accept the offer within 15 minutes you can click the "The buyer didn't accept my offer" button to lock the listing. If you do this, please make sure you have cancelled the Steam trade offer you sent.
Once a listing is locked, a member from Staff will be needed to review your listing to solve the dispute before you can create or purchase listings again. If you want to speed this process up, feel free to contact Support.