I'll have to refute this method.
After killing numerous
Great Turtle Furyshell - NPC - World of Warcraft on the beach in the Timeless Isle, I had a Burden drop. Immediately, I advertised in vicinity chat WTS Burden, with a Death Knight that did participate on the kill, from the same realm as me, trading me. He was not partied w/ me.
I added the item to the trade, in the trade slots, and it automatically moved in the Soulbound slot. Manually dragging the item up brought the usual "You can't trade soulbound items." error... This occured on Silvermoon EU, so I have no idea if they fixed it or not, but this was my experience.