If a buyer doesn't want to complete a transaction they just don't pay. In that case it ties up your merchandise and makes a mess of trying to keep track of. On the other side I had a customer whose purse was stolen and all her credit and debit cards were cancelled. It was going to take at least 2 weeks for her to get things straightened out. She just didn't want anything more to deal with and I sure could understand how she felt.
I would like the ability to cancel transactions for a customers reasons or my own.