This is not my finding, and just about entirely copy pasted, and has already been reported on official Blizzard forums. Good luck with going at it, though. I'd probably expect a ban if you do, so shuffle whatever profit you make quickly.
Video of exploit in action
Warning: Bad music and in a non-english language
Appears to be fixed now, approach with variation in steps mentioned, feel free to drop any new information if you find a way to get this working again.
Here is how things flow.
A: person with resources
1.A gives resources to B
2.B crafts items using them.
3.B gives crafted items to A
4.B moves to the new save point or waypoint
5.A leaves the room (When A leaves the room, A loses all crafted items received from B, but gets all resources back as well. Therefore, A lose nothing and gains nothing)
6.The room automatically terminated (server detects anomally?)
7.At this point, server lost the track of B giving crafted items back to A (from step 3), thus B still has crafted items. Therefore, B freely gets crafted items. (Because, A gets back resources)
1.B crafts items using resources received from A.
2.A exits, and his inventory resets to the original state.
3.B still has crafted items
you have to be in a difficulty or area that person A has not been to yet. that way when you leave, the game resets and kicks all players.. this will not work if you are both in the same difficulty, it will just keep you in the game instead of kicking you. the best way is to have someone on a lv. 1 character do the stash trading of resources from like.. the end of act 1, that way when the creator leaves.. the lv. 1 is kicked out afterwards.