There are a few ways to go about this, depending on what has actually happened.
If you have lost L$ then it is one of two, the money has gone through to the seller, the item has been sent, and the server has registered the transaction as succesful. This could be due to you accidentally cancelling the item delivery, or due to a server error (which is unlikely.) In either case, the transaction will have been saved within your transaction log which is located within your dashboard. Contact the seller and copy the transaction details for them, to see if you can resolve the issue with them.
The other possibility is that the server has recorded the transaction as failed, but it some times takes up to 8 hours to refund the money. If you still haven't received the money after 8 hours, the best you can do is to fill in a support ticket for the Linden Labs and they will look into it. There is no quartanee that they will be able to help with issues like this, depending on the situation, but it's worth a shot. Just detail your transaction and what has happened to the best of your knowledge.
Since I am not sure what has exactly hapened the question is difficult to answer, but I hope this will help you get started.