Redditor Vernm51's parents couldn't find a copy of Pokemon HeartGold for the DS locally for Christmas, so they ordered it online from GameStop — and received a bootleg copy that didn't work.
The cheap Flash card-based bootlegs that plagued the original DS don't work anymore due to 3DS firmware upgrades — they register as THQ's Alex Rider Stormbreaker game and then fail to load — but there are still many in circulation, like this one that unfortunately ended up under Vernm51's Christmas tree.
Several awsome Redditors offered to get Vernm51 a real copy of the game, and eventually a local GameStop (featuring the guy with the tattoos and Darth Vader shirt) got him a copy of Pokemon SoulSilver. Still, what a disappointing thing to happen on Christmas, both for Vernm51 and his parents, who remembered how much he loved playing Pokemon Gold as a kid and wanted to bring that joy back.
This is not joyful.
If used is the only way you can find your favourite old game, then by all means grab it. Just try and grab it locally, where it can be tested and easily returned. It's not just DS carts — it's easy enough to print up a fake Blu-ray disc label if you've got the right tools.
There's only so much checking a game retailer can do. The warehouses of a major US chain like GameStop have been sitting on inventory so long that the fake copy of Pokemon HeartGold they shipped out might have worked at the time it was traded in.
If you must buy used and must buy online, look for eBayers with good feedback. Their reputation depends on making sure their games work. Otherwise you'll never know when you're going to accidentally ruin Christmas.