In the picture I posted below, imagine I placed one refined metal instead of 10 CE.
So basically, its one refined vs some CE for example.
When both of your items are ready, you place a check on the "ready box". But after you click the box, a pop-up message would appear saying that "..Your trading partner has not offered any items in the trade and after the trade is completed you will not receive anything etc".
The pop-up message appeared because my trading partner only placed CE on his trading slots and nothing else. This should not happen in my opinion as it appears to me that he was trying to scam me.
CE is a currency as listed under Steam. It's like giving free money, and Steam says, "Woah, are you sure you are giving it for free?" Anyways, I don't see anything wrong, it is not saying that it is a scam, rather, just a reminder to tell people that, yes, you are, giving free money.