As of now, trading (outside of stalls) is done entirely on the honor system, you have to trust that the other guy isn't going to grab the crate and teleport away without upholding their end of the deal. I realize that the devs may actually want us to be able to do this, but otherwise, here's what I'm thinking.
When you right click on someone, among the other options would be one that says 'Trade'. You click it, the other person gets a window asking if they want to trade with you. They click yes, you approach each other, and a window appears. It looks like the normal inventory window, except with 2 sections. You put items from your inventory into the section on the left, and can see what your partner has put in in the section on the right.
At the bottom of the window there would be three buttons. The first is 'Trade', which you click when you agree with the swapping of what is in the two windows. When both players have clicked it, you can then transfer the items to your inventory from the other window, but can't touch the ones from the left now.
The second button is 'No Deal' or something to that effect, which cancels your having clicked the agreement button. This is for when you've clicked it accidently, or the other player has changed what they put in their side after you clicked 'Trade'.
The third button cancels the trade and puts the items back in your inventory.
This will stop dishonest traders (apart from those dealing in carry items), who don't leave a scent.