While I do really like the idea of a marketplace where we can place items for sale using in game system. I can't stress this enough this is probably the best post I've seen about a system similar to an auction house being added to Nin..
I can tell you this probably won't be added, and if it does it's for a long time from now.
Rory has stated many times he doesn't want an auction house in-game so your idea of having players meet up would be great (maybe even making it a gold-sink 5% or something)
The major problem I see is that this is essentially adding what the discord #trade does to the game (which in itself is great, but when you take into account the coding time it takes on an indie dev team.. it's very far down the priority list.)
Anyway like I said, the fact that your suggestion is really fleshed out shows you put some thought into your idea rather than just "gief mmo auction house nao plz"