Safe trading system set up for expensive trades. Is it possible?

I had an idea that I wanted to know if it would be useful, considered, and something that is able to be done given the game's files and whatnot.  It's more than obvious that in order to get what you want that expensive trades are necessary, but, then again, the fear of keyloggers loom.

Would it be possible to set up a function that allows two players that want to trade through npcs that allows them to each put in an amount and/or item for said amount and lock that in place via a drop down box or something simplistic  Next each player must put in the items on each of their ends to meet the locked in function.  Once the obligations have been fulfilled, the item and/or dil is sent to them automatically through the mail system, automatically in each character's stash, or other method?  If this isn't possible then disregard the suggestion as I am not up to speed on this program's limitations or whether a developer could actually do it.

Thanks for taking the time to read.