I only had an issue with DLC on eBay once. Some guy bought a brand new code for $5 and said it was used already. I called up Microsoft and they said that it was redeemed the day the guy said it was used. I didn't feel like risking a negative feedback over a $5 code (since I would lose more money on sales in the long run since I was at 100%) so I just gave in to the scamming asshole and refunded him. Soon after that he gives me a neutral feedback. Not only did he scam me, he also felt the need to slightly hurt my reputation.
Another incident was over some dumb foreign guy who bought a code that was specifically for the North American market so would only work on the North American PSN, and then got pissy when it didn't work on his European PSN. He was definitely a grade-A jerk, but I refunded him and at least he never used the code so I was able to quickly sell it someone else.