Bought a large item of equipment from eBay, never been used and was described as having no damage. Found a small defect which needs repairing before it can be used (won’t work without it). We’re happy to repair it. Seller has been great, responded quickly to apologise and offer a partial refund. They’ve asked how much I’d like back…. What percentage is reasonable?! 10? 20? They’re a business that deals in selling manufacturer returns, if that makes a difference.