Unfortunately, it’s sometimes just not possible to keep everyone happy. In this case, the mistake might have been on my end, as a coin that had already been sold through Ma-Shops somehow showed up for sale again—I honestly don’t know how that happened. The buyer was informed immediately and got a full refund right away, even though he had used a discount coupon of 20% he received 100% instead of the paid 80% back for this particular coin. I understand his disappointment, and if he chooses to leave negative feedback, I probably deserve it from his point of view. That said, I also think it’s not very "professional" to leave negative feedback without first reaching out to the seller to look for another solution.