They could have some gear or other stuff that costs a lot and gives you a lot of money. Or certain quest choices where you can help people out with your vast fortunes. There are tons of ways you can give both kinds of players what they want.There is a mod that let's you reduce the sell value of loot by a percentage. It only works one way --it doesn't reduce the buy cost.
http://www.nexusmods.com/witcher3/mods/6/?
In my game I have it set to 60%, but that's still too high, so I am thinking of switching it to 40%.
Part of the problem inherient in this issue is how much money you have partly depend upon how much you get side-tracked. If you're someone who explores the world a lot you will have more money than someone who only does the main quest. If they balance it for ther latter type of person than the former has too much money, and if they balance it for the former than the latter doesn't have enough. The game almost needs an adjustable economy slider, which is the mod linked above is so handy.
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I think this could've been solved by smart loot placement and limited inventory. Why do peasants have dimeritium ore, quicksilver solution and other expensive components and why are they not selling these things? They are just way too many useless knick knacks you can pick up, candles, rakes, broken oars, fishing poles? Why? Why did they fill the game world with so much useless stuff and why did they make so many searchable containers. I would've preferred the TW1 inventory system to be honest, you couldn't carry 5 swords and 5 different pieces of armour, at most you carry one armour or one sword and that was only if you dropped a ton of other items. I also think it'd be better if more items had lower caps, so you could only carry like,30 of each alchemical reagent and 10 crafting items.
Caps are annoying as hell. They should just make being rich worth a damn. Oh yes, and less loot in general. The main problem is that finding treasures and receiving "legendary gear" as quest rewards isn't rewarding at all. It is just more junk to add to the list. There is no incentive to explore besides exploration itself. All the stuff you find you sell anyway and then you have money you can't use for anything meaningful.