trade in your items to get creds for other items?
Yep. You basically sell certain items to Warehouse bots for a certain number of credits which are usable only in tf2wh.com. Afterwards, you can buy shit with the credits.
They do, however have quite a bad reputation from lag and such when customers are interacting with their bots (takes to long for bot to send the friend request, the bot does not give back unusuals, etc) -- For items such as unusuals, the user has to set the price and there is a fee to set it. (That is why when some people are not able to have the bot sell it, they want it back but due to the server stress, they cannot get it back until quite a while)