The thing is, only in a narrow set of situations is this even a "problem". As long as both players have a legit reason for rolling on the item, then there's no real problem with them trading it back and forth. The problem only comes out when one player rolls on something they don't need just to give the other a better chance.

I'll admit, I run with my wife (an Ele Shaman). I would *never* roll on a piece of caster mail (as a mage), just to increase her odds of winning. On the other hand, we both roll on caster jewelry and if I win, she might be able to convince me that she needs it more than I do. The same is true with things like mounts or really anything else that we both can use and don't already have. As long as that's the case, there really is no problem.

The simple solution to keep it from happening in the cases where you don't want it to and allow it in the cases where it's OK is this: Pay attention to who rolls. If they can't/shouldn't use it, then don't let them roll. If they can and then want to give it up to another player, that's their prerogative.