Swap your duplicates
Pick what you give (duplicates only, by default) and what you ask for, and send the link to whoever you like. When they accept, the swap happens in both collections at once. You need your collection in the cloud.
Orbital market
First, your collection in the cloud
To trade, both people need their collection in the cloud: that way the server checks the cards exist and makes the swap in both collections at once. It's free, no email or password.
Save my collection in the cloud