The "bug" that caused the Riverwood Trader to have ~10k gold to trade was fixed by the latest patch. It would be great to undo that. It was nice to have a place to sell high value treasures, some of which are valued more than the 4k of fences, which not every player unlocks anyway.

In my view this was not unbalanced since there was only one such trader and there is otherwise no way to sell good quality armor or weapons found in dungeons, especially if you temper them.

Sure there is. You just have buy goods from a vendor to build up their cash. Saving and reloading from desktop(new session) will reset a vendors stock and cash so you can resell everything you purchased without waiting. Also if you go to Warmadiens's in Whiterun and deal with Adrienne outside first before going inside you get a double vendor stock and cash flow.