This is the situation in Scotland (I don't know what applies in England): if the purchaser fails to collect the goods, the shopkeeper can regard them as having been abandoned. The shopkeeper should let the police know that the goods have been abandoned. The police have a responsibility to take charge of abandoned goods, and to keep them for two months. If nobody gets in touch with the police during this time, at the end of two months, they can: return the goods to the finder (and that means the shopkeeper who reported them as being abandoned); dispose of them in a sale.
If the police return the goods to the shopkeeper, the goods now belong to the shopkeeper. If the original purchaser comes to collect the goods within a year of abandoning them, they still have to be returned to the original owner.