What do you do when Steamtrades fails you?

Try to be explicit in your thread title on SteamTrades. It takes time, but people will post their offers.

You go on Real Steam Forum there is Trade I used it for coal on Christmas.

I don't use it, because of that.

How long are you waiting? It can take time. People are usually after the biggest, newest game, that's no surprise and it'll be the same no matter where you go.

It's normal, try instead of wait for a offer, give a offer to someone.

I usually curl up in a ball of rage

Try to be as explicit as you can in your thread title and try to regularly bump the thread (with the button, and only once per hour).
Even then it can be a long wait, depending on what you have.

I posted a thread about CS:GO there, immediatly got 3 responses.
Traded ir for RS+RO2.

just wait.. while you're waiting, go post some offers in other threads, even if you know they will refuse (because others visit the thread and will see your offer as well. that's actually how i get alot of trades). several times i've posted in a thread, the thread owner would call me a lowballer, then someone else added me saying they wanted to do the trade :)
you can also go in Traders Guild chat and post your offer there.

The best thing you can do is wait, if you have the patience. Someone may not be looking for the game you have, today, but tomorrow, who knows. I had a Sleeping Dogs key from the Nuveem sale that no one wanted for a good week, then one day, someone wanted it, & I was able to trade it for the game that I was looking for, which was Don't Starve. So, yeah, patience is the key, if you have it.

THing you have to remember. GAme keys are worth less then their giftable counterparts, and the price of a game is valued at its lowest price. For example, CS GO has been as low as 7.50. So its not worth 15, its worth 7.50. You mentioned L4D2. Its been on sale a million times, lowest is likely $5. So thats all you will get back for it. Ea key is worth aprox 1.88 (can be up or down from this) so you can expect about 2.65 keys or 3 keys.

Use search for similar offers and offer there (but don't steal a thread or something like that). Make an offer on Official Steam Trade Forum, Redit GameSwap, wait...