Selling game/item codes is a grey area on eBay. eBay restricts the selling of digital goods to approved sellers, but it is theoretically possible to circumvent this by physically mailing codes to buyers. The problem is that this side hustle seems to frequently result in chargeback/return fraud, wherein buyers purchase codes and then claim they don't work.

I've sold ~$50k worth of items on eBay over the years and have had a mixed record dealing with eBay's customer's service. Unfortunately, eBay tends to side with buyers, leaving sellers prone to various fraudulent schemes by buyers. However, they sometimes will protect sellers. For example, I sold a Panasonic Toughpad that a buyer spuriously claimed wouldn't connect to cellular networks (I tested the tablet beforehand and everything was working properly.) The buyer attempted to extort me by saying he would leave a positive review if I gave him a partial refund and implying he would leave a negative review if I didn't (this is a big no-no on eBay). Given that I had written evidence of this behavior, customer support sided with me and closed the case and removed the negative review the buyer left. Not every situation will have a fortunate outcome like this though.