Whenever Nintendo hands out event Pokémon (special creatures that players cannot catch in-game), it sends pre-programmed cartridges to participating stores around the country. In an effort to keep the number of legitimate monsters down and make special events at all relevant, those same stores must send the distribution cartridge back to Nintendo pretty much immediately.
If you want to start up your own Celebi-farming business, you can buy this distribution cart for $41. The daring Poké-thief seller will even throw in free shipping to the auction winner.
Judging by the wealth of such items on eBay, almost no one listens to the Nintendo representatives who occasionally swing by stores to hand out goodies and remind all the employees how awesome the company's products are. That's surprising, since I've seen those reps get pretty hostile over free Pokémon.
From my limited time working at the now-defunct Game Crazy, I know Nintendo is pretty fast to demand that all event cartridges are returned in a timely fashion. Once, my store was a day late getting ours sent out and the company's sales rep showed up to take it herself.
Kind, gentle Nintendo means serious business when it gives out free stuff. I imagine the seller is trying to move this cartridge for fear that Nintendo will disappear him if he holds onto it too long.