How do I use amtv?

Recently got into trading again, and had heard of amtv for offers so figured I’d try it out. I’ve accepted two offers so far but both times I’ve added them, waited a few days, and then they’ve marked it as failed due to me? I’ve triple checked both my and their users on the site and was careful not to accept anything that looked too good to be true. Am I doing something wrong? Haven’t touched the auctions or anything because I’ve heard about them, but I thought normal trading would be alright?