If you can see old market prices, I don't think they are applicable anymore (like you surely won't get anywhere near 5000 gems for them, which would be around 1.5 dollars)
I made 3 screenshots to show the steps (attached to the post), you can start directly on the trading forums for the game since you have shown that it's about Wyatt Derp, so here
Wyatt Derp Trading :: Steam Community . If you want to repeat the process for other games, just follow the steps in the first screenshot to get to the forums for the game (make sure you're under Trading section once on the forum)