To keep matters more simplified, I've done some research and found something that might be helpful. This guide should help you delete a TX that was created with Bitcoin QT. Keep in mind that you should backup your wallet before trying this; also if you're receiving a transaction (and it has been unconfirmed for a while) you can't really do anything about it.
AFAIK the expiration of transactions isn't a feature of Bitcoin as they don't have a time field.
Well, thank you. To keep it short and simple:
I am the seller and i am waiting a payment, i am using Coinbase.
The buyer use blockchain.