How to trade for TF2 Keys?

I don't trade but looking at official stores (ITAD) for price ideas won't do you any favors in trading.

Was it bundled? If so, how often? How often does it go on sale and at which price? Is it a rather good game or not?
Check SteamTrades and barter to see how many people offer the game, how many are looking for the game and whether some of the traders have set prices.
If you click on "8 risks" the risks are explained.

I do not trade but you can try to offer the game using the official store price with a decent discount (30% or more); or around the historical low. If no one want to trade drop the price a bit more.

The risks on keyshops are listed here: https://gg.deals/page/keyshops-risks/

I used to trade a lot; I still do although a bit less nowadays.

If you want to trade with a friend or someone you know, then you will have to come up with your own way to approximate a game's value.

To trade with random internet people, you can use SteamTrades or barter.vg (or lestrades, but it's seeing less traffic). On SteamTrades and especially on barter.vg, most people use grey market price sources (such as gg.deals). In my opinion, if you use a median of the first 3 lowest gg.deals keyshop prices, both parties should get a good deal. Be sure that the version of the game you are trading is the one being listed on gg.deals. Sometimes a version is region restricted, has VR capabilities, etc.

You can look on barter.vg for past trades containing the game you are looking to trade. That should give you an idea of how people evaluate that game. Also, note that there are some resellers (on SteamTrades and barter.vg) that will always try to get games at a lower price. Otherwise they simply cannot turn in a profit, as they are limited in terms of how high they can set their selling price.

If you have any question, don't hesitate to ask. I've been scammed a few times and it hurts every time, so I'll be glad if I can help to prevent the same thing happening to you.