Hi um, I've been trying to buy 2 copies of Warhammer 40k Dawn of War 2 from this site:
and it continually kept failing repeating the same error popup message each time "Dispenser.tf is under maintenance, please try again later"
Pictures here:

I kept trying over and over for hours and I got a bit frustrated and tried to buy something else to test if I had the same problem with other items, and to my surprise it went through and I came to the conclusion it had to be those individual trades that was the fault.
So I tried to cancel the test purchase I made to receive my keys back so I can try again later, the order canceled but I didn't receive my keys back, I presumed it would automatically return the keys as I never really collected the game I bought to test.
Unless I'm missing something, and if I did, I apologize for the inconvenience, I'm still getting used to this site.

Because you are not accessing with "https" when you buy Warhammer. If you access with https://dispenser.tf/market/15789/ it would work. I have also reverted your test transaction.

We are going to implement force conversion of http to https very soon seeing this has become a widespread problem.

Just as what I have described, you may not be accessing the site with https so you could not send request to our server.