I certainly wouldn't, personally, be a customer in this market, but I'm sure there are people who will pay for anything. However, I do find that this doesn't already exist to be a red flag, in the sense that there probably isn't a market for such a thing, as solo players would have "monetized" it long ago, if there were.

I think it just comes down to the fact that, as you, yourself point out, it's pretty easy to just purchase crafted pieces of gear that have an item level of 400, and after a few days it's expected that WQs would just fill in the missing Azerite slots. Rings, trinkets, and a good weapon have always been the most difficult to obtain, but even then, WQs and Emissaries will continue to give gear upgrades, and it won't be long until a player has a high enough item level to begin LFR and Heroics after a week or so, assuming they're not already buying Mythic dungeon runs.