Its most likely items that were crafted before the update was made. I wasn't around before the update but my guess is that there was never a level requirement (and thus no binding) to make 4* and 5* gear. Its either that or there was no binding back then. Someone who played for a longer time then I did could help you out with that but the reason ultimately is because the gear ws crafted before the update.

Just be playing a few months ago before the crafting changes, which more than doubled the cost of crafting and made it so that bound items are bound on upgrade. Or even more recently, when there was no earlier item and you could directly craft stone tortoise and a few other items (every item now has at least a 3* item before it in the crafting path, I believe, though Fang of Vog is its own thing.) Also you can get really, really lucky and find one. Items -do- drop from chests occasionally.

I wish I hadn't been so hasty to sell the Ascended Calibur (4*, no UV) that I had... I was expecting the unbinding service to come much sooner.

(Sniped by much speculation! To clarify, before the crafting update, you still had heat requirements and 3* items were bound before you crafted the 4* item, HOWEVER, every newly crafted item was unbound even if it was crafted from a bound item. In this way you could unbind by upgrading.)

A couple of months back, crafting was different. When you made a 4* item, it still used a 3* item, but it would become unbound. If anyone has those items still in their inventory, they can sell them at the AH.

Additionally, a few 4* and 5* recipes didn't use lower level items until recently. The Graviton bomb, and the Heater shield are a couple of examples. So people could make those items and they wouldn't be bound upon creation.

So from time to time these items will show up, but it's unlikely that anyone has any way of creating *new* 4* and 5* items to sell.

Edit: started writing this post, got distracted... now that i posted it i'm tripple-sniped.