• Go to your items
  
• Go to your backpack
  
• Right-Click on the crate you want to sell
  
• Click on "Sell on Steam Community Market "
  
• Click on the "You Recive" box
  
• type in 0.01 and press enter (do not do this with other crates other than the normal ones halloween smissmas etc. Price check those types of crates on the steam market before selling them, and sell them at the proper price)
  
• Check the "I agree to the terms of the Steam Subscriber Agreement" box
  
• press the "OK, put it up for sale" box
  
• confirm on your steam app

the only reason this looks like a guide for stupid people is because, when I had around 500 hours I have never even heard of the Steam Community Market

if your lucky you can sell 1 or 2 crates on the market and buy yourself a pretty little emoticon or something (crates don't sell often but, it's better to do this than delete them or keep them (or even unbox for that matter but hey, you can do whatever you want.)
and do not do this with other items that you care about for storage.
You can also take it down the market whenever you want to get your crate back.
if you don't have the steam app you will have to wait 15 days before it is on the market