Just a few thoughts.
... Despite being a long term Pathfinder player (until the last few years) I found magic mart a real turn off. I don’t like what it does for cookie cutter PC design and builds.
... Smaller Baldurs Gate style shopping lists seem fair enough, but for me shops with 100,000 go stock items break verisimilitude.
...If I want to see where PCs can spend their ‘winnings’ I look to other games for inspiration. Skyrim in particular.
1. Commissioning or crafting items
2. Good causes, e.g local orphanage
3. Spell casting services
4. Property (might be a home in a city or a business)
5. Art objects - statuettes, paintings, luxury goods etc
6. I’ve adapted masterwork item rules for amazing quality weapons and armour - a masterwork item add either + 1 to hit (finely balanced) or +1 damage (exceedingly sharp) at 500 go each ontop of the cost of the weapon and doesn’t hurt creatures affected by magic weapons.
7. I took the rare materials from Magic of Faerun and converted them to 5e. They are available for purchase at a cost.
8. Learning new skill/Took proficiencies.
9. Rare components
10. Discovering new spells.
11. Living a noble lifestyle - hosting balls, gambling, boxes at events etc, sponsorship of competitions.
12. Sponsoring junior adventuring parties and rewarding them with gold / treasure.
13. Ploughing into an organization (their temple, fortress, or guild etc)
14. Performance enhancing drugs (Lords of Darkness)
15. Poisons
16. Alchemical research.
17. Patron of the arts
18. Political aspirations (or support someone else’s)
A lot of these are things that rich people do these days or have always done throughout history. Of course a lot require a bit more work as the DM or Player but to my mind are extremely rewarding. I think Skyrim does it better than any other game I’ve played and I well worth a second reminder of how they progress these things.
[Edit]
19. Bribes for criminal activity
20. Paying for housing and feeding of followers
21. Researching a particular problem/place/people/monster/legend
22. Travelling the world... the grand tour
23. A collection of a particular type of item (elven glasswear, dwarves warhorns) or of more practical items - weapons of all different cultures (think Yu Shu Lien’s dojo in Crouching Tiger Hidden Dragon)
24. Amassing a library or alchemical laboratory.
25. Outfitting a ship
So many more things people could spend money on. These seem infinitely more fun than a DM deciding ways to take money away from a PC through theft, treachery etc.