Mellie, what are you going to do in 3 weeks time when the original buyer comes to you wanting their item?

Until that contract is cancelled, you still do legally have to honour the contract.

As Basil said, you really need to open up an UID on day 4, then hope like crazy that they don't pay in the following 4 days so that you can close the dispute which cancels the contract and leaves you free to list again.