Equipment degradation.
Every item starts with a d12 as it's quality die.
After every encounter, for every piece of equipment used rolled, roll the equipment's quality die. If it's a 1, it degrades down to a d10, or a d8, or a d6, or a d4. If you roll a 1 on your d4 quality die, the item is no longer functional.
Money can be spent to repair the quality.
That is a lot of work for not much payback, and one of the first things I think items would be enchanted to deal with. Even in a game about resource conservation, I think it would be better to roll up it all into lifestyle expense, fumble mechanic, or some occasional mechanic somehow.
As for magic shops, besides storefront wizards or temples that might have some stock scrolls or spells on hand, the main magic item dealer was a broker, somebody that knows who has what for sale, who wants what, and who can make what. They vet everything, get people together, and get a percentage of the deal. In my last game they were also a lawyer that dealt with such deals and the PCs used it in a reverse function. They were chasing an evil wizard who was probably hiding with one of their noble connections in the city, the dug through their magic items for things to temp said nobles, and the broker get them inside said noble states so they could question and look around without breaking in.
After reading an article on the
Medici Bank, that there are 'great banks', which besides doing money changing, letter of credit and other such bussiness, are basically pawn and loan for nobles and other rich people. It would deal in luxury items, jewelry, objects d'art, gems, bullion, etc. They would be the people that buy magic items at half value quickly when the PCs want to sell them. They probably also would have some to sell. Of course, failure and loss of everything was a very real possiblity.
As for the OP, if all the PCs are doing is drinking and such till their next adventure and never looking at doing something grander, then there is not much need for all that money. Certianly it has been stated there is not much to do with money after a point anyway. Magic items are found. Lifestyle expenses as normally within reach for what PCs can carry in their pockets.
As for what to do with late game money, Bastions are a thing now. These seem to take the place of a home base. There has been full domain management in the past, but those often get into complicated systems that people don't want to deal with and have annoying flaws. For 5E, I expect greater domain management should be like magic items or bastions (if not rolled into bastions all together). PCs go an an adventure to set up or take over a domain. Upon success, it gives them a chance to spend lots of money on getting one. These grant some other benefits, but this also begs to be a different tier of play as one moves armies around and such. Still, things could be kept more story than war game.