I recently had a dispute with a business selling products on Allegro. They refused to refund full price after I sent back the product. They claimed that my unpacking of the product to test it made it lose almost all its value because it would be difficult to resell.
The customer ombudsman sided with me. So did the Allegro Buyer Protection programme, subsequently refunding me money. I'm still not fully satisfied - Allegro told the seller they were not expected to reimburse the money themselves.