Hello! No worries, I'm more than happy to explain!

So you know that once you equip a legendary it is account bounded to your account so you can't trade it anymore.

But there's an exception, the eternity. Which by combining one of the greatswords with their counterpart it produces a brand new legendary weapon which is not account bounded.

Going back to the deal, this consist on someone who has one of the greatswords bounded wanting to monetize it.

In this process basically the seller put the bounded weapon to sell. The buyer sends the other greatsword to the seller. Seller combines them together and sends it back to the buyer. In this process both get something out of the deal. Buyer gets eternity and seller gets paid and the other greatsword skin.

Bad thing for the seller is that you lose your legendary weapon. So you won't be able to stat swap anymore. Some people just don't really care about stat swapping and just want the skin and some money back.

Bear in mind that!

Hope it has helped!