do the shopkeepers like Ribaldi (??) in the Promenade ever change their inventory? or do they always have the same thing?

maybe

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After you come back from Stronghold, or Underdark, Ribald changes his stock.......or might not. But He definitely make new stock for special items that you need to pay to see them.

Bernard(?), the shopkeeper in the Copper Coronet has a few new items after you help replace the management ;)
Ribald has a new set of items in CHapter 6.
There also is a guy in trademeet who either gets new inventory, or begins selling goods, only after you have finished the main quests there and become the Hero of Trademeet.