Saver's remorse
The feeling of regret after one has controlled their impulse to buy something (usually expensive), and then later returned to the store, only to find that their intended purchase has already been bought by someone else. The opposite of buyer's remorse.
Jack had serious saver's remorse after he returned to Nordstrom to buy a sick pair of Rock and Republic's he saw a few days ago, but found that they were completely out of his size.