I think from what it sounds like you are aware that the other type of weapon/unit crafting is still not in. So your saying is you want Weapon (A) with certain augments/skills that you pick. And you want that person to then open a trade system with you to buy it at a negotiated price?
I think for that to work you find someone who is willing to get a weapon you want, example Psyco wand.
You tell them what you want on it:
Get an element you want, let's say wind,
Get a soul augment, plus mutation 3, plus meseta fever.
Once they hear what you want they tell you a negotiation price, considering they have to take into effect what they put on it, how much meseta they waste, the fail vs sucessding.
And then they sell you the item.
This shouldn't be that hard, I don't know many people that consider themselves crafters, but you would need to have a premium account so you can trade with them. Also I would think that they would also need to make it a certain amount in case the buyer backs out of the deal. They still have the item and can sell it in the personal market. But before you do that, if money isn't an option, you can search the weapon, unit and the element, and augs in the search bar to see if someone by luck already has it, and you can negotiate a price with them. Might be easier.