NOT Auction House trading, but in-person trading of bound items following these rules:

You may trade a bound 0* or 1* item, provided the opposing offer includes a bound 0* or 1* item as well

You may trade a bound 2* or 3* item, provided the opposing offer includes a bound 2* or 3* item as well
You may only trade/receive bound T2 items if you have T2 access already.

You may trade a bound 4* or 5* item, provided the opposing offer includes a bound 4* or 5* item as well
You may only trade/receive bound T3 items if you have T3 access already.

What does this accomplish?

You would still have to achieve T3 status by crafting a 4* set yourself.
You could trade bound items of yours, with restrictions, but not sell them on the AH.
You could in effect craft 4* and 5* items for friends, but only if they already have T3 access & they have a junk bound 4*/5* item to pass to you.

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Notes:

Yes, I know it's an awkward idea, but it's meant as a stop-gap (temporary) measure , until something better can be formulated.
It still protects access to Tier 3. I figure it'd be better than nothing, no?

And yes, there will be a lot of 'paper weights' being passed around, but if a player wants to give away a bound item to a friend for next to nothing, I think that should be their business. The biggest thing is players would still have to craft their way into T3, unless they can really have a friend find a full set of T3 items from drops.

Edit: trading bound items should reset Heat levels to 1.
Edit 2: may be able to just make it Tier-access dependant, removing the 'paper weight' aspect of the idea. Assuming AH is prohibited still, all the rule needs to be is having relevant Tier access prior to trading/receiving items of that Tier.