How does trading work with TF2 keys?

This isnt a trading site but here is the listing for tf2 keys and maybe you can work it out from there https://steamcommunity.com/market/listings/440/Mann%20Co.%20Supply%20Crate%20Key

First, this is not the right place to ask. There are trading sites with forums, or even hell ask one of the people you saw the offers in about what it is and I am pretty sure they would have answered.

But as for your question, a TF2 key is worth roughly $2 in paypal and with xitau's link you can also directly buy some keys off the market, but they would have a 7 day trade hold. Also after the trade hold, there will be more if you don't have steam authenticator on. I advise if you never tried trading with steam market items before to just find someone selling it for paypal, or ask the seller if they would like to trade with paypal instead.

https://www.steamtrades.com

https://barter.vg

Better places to ask, there it is allowed to trade and discuss this. Here it isn't.

Also: please read something about safe trading guidelines, because paypaling or giving TF2 keys to a private person has some risks, because even with TF2 keys you have to go forward with nothing guaranteed in return. Because they send the plain text with the Steam Key afterward or they could scam you.

Also maybe Group buys is something for you or grey market / steam key resellers (no sources here allowed), because most bundled keys show up for a cheap price there and that is way more safe. (still it has some risk, but i never had problems)

Edit: For me it was never appealing to buy TF2 keys on Steam and to use it as a currency, dunno seems odd to me. Money is usually fine and convenient :D