This is likely a mistake or an error on their website.
Here's why:
* HeartGold and SoulSilver are old games: They were released in 2010, so it's unlikely they are brand new stock.
* Gametraders specializes in new releases: They are primarily known for selling newly released games.
* Pre-orders are common for new games: If they were going to sell a new game, they'd probably have pre-orders available rather than selling the game outright.
It's best to check the Gametraders website directly to confirm the situation. If the listings are still there, you could contact their customer service to ask for clarification.