Make a new add-on that is a mini auction house station. It could use the same slot as the vendor, and even re-use a similar asset. I could see it being a nice convenience item for the store without being in-balancing since auction houses are everywhere.
It should be a different auction house tho. Like a black market..somthing different about it to make people wanna spend their gold there instead of the starter city.
Zero market fees when accessing the market thru a house vendor would be nice. Taxes would offset a little of the gold sink, not all. Just a thought. Houses need some love.