Does Waterdeep(population 132,661) have the same 100,000 GP limit as the 25,001 population metropolis listed in the DMG? It would be rather.. odd.. that it should have a GP limit the size of a town a fifth of it's size.
I would say that with Waterdeep, if it's for sale any where, it's for sale there.
That said, I won't be allowing anything worth 100,000 gp to be sold any where in my campaign. If someone wants to commission a 100,000 gp item, that's a little different.