Two things drive the use of resources, and they have to be balanced: scarcity and necessity. If resources are scarce enough (like Malachite gems in Divinity 2), that promotes stockpiling, even if using them could be helpful. If resources are necessary to survive, players will use them even if they don't want to. In Divinity 2, resources like Dexterity/Intelligence/Willpower potions weren't really necessary, so they ended up getting stockpiled because of lack of use. In my recent playthrough, I had to force myself to remember to use the potions - not because I needed to, but just to use them up.
Intelligent resource management can be a great addition to a role-playing game. Generally, I think that the early stages of a game when you can't buy everything are much more interesting than the endgame when you've got loads of money and nothing you really want to spend it on. Becoming the richest person alive might give you the satisfying feeling that you've achieved something in the game, but it can also lead to boredom and frustration: The player might not bother to smash or open containers or to pick up loot anymore as the chances to find something useful are so slim. And selling stuff that you don't need is almost pointless when you already have twenty times more money than the most expensive items would cost.
Players not opening crates anymore is the fault of a loot system that isn't balanced right. It can't just have random generic items, all appropriate for your character level, it needs either some pre-placed very useful items, or better yet, a kind of high-class random item that is more powerful than most things you can find or buy in stores, but only has a small chance of appearing in random crates.
The difference between unique items and high-quality random items is that the uniques are all crafted for a purpose, all its attributes are focused. High-quality random items have very good attributes, but they're randomly selected.
I think that both inventory space should be limited and traders should only buy limited quantities of any item before prices drop, even to 0. That way, it might rarely make sense to run back and pick up stuff you couldn't carry the first time. And it could easily be explained in-game: "Another orc helmet? Sorry, but I still couldn't sell the last 20 ones you've brought me. I simply don't have any orc customers, and collectors of such trophies are rare. Maybe you'll get a few coppers at the scrapyard, though."
A formula to reduce the amount of gold you get for every identical item after, say, the fifth one is an interesting idea, but it has some drawbacks. Some players are compulsive loot picker-uppers, and making you switch merchants just lengthens the amount of time they'll spend running to different merchants, instead of playing the game. This will probably start to get annoying sooner rather than later, and annoyed players aren't happy players.
At first, I also found it hard to leave loot behind in The Witcher, but in the end I was glad I didn't run back and forth umpteen times just to make some money I wouldn't have spent anyway. In most RPGs, one ends up with tons of money and very little left that is worth buying. Truly limiting money as a resource and thus making it impossible to buy all of the best items might be the base for some interesting (though hard) choices in the late game. If heroes can't achieve everything, e.g. when they have just two curing potions for three sick people, why should they be able to buy everything?
Limiting the amount of money available is one idea, but I'm more in favour of late-game useful items that cost a lot of gold. Divinity 2 was pretty good in that respect, because merchants sold unique weapons that were better than anything you could craft or find randomly, but they cost a lot of gold. It acted as a gold sink.
It's plausible that the number of items characters can carry is based on their strength, but it's also a big advantage for those who consider strength their main attribute anyway, i.e. warrior-types. It would be nice if this was counter-balanced by other stats also having a non-combat use - intelligence could enable certain clever dialogue options, for example.
That'll work, as long as you make it clear in the description of the attributes what they do. The bar for attributes opening conversation options would have to be pretty high as well, and get higher the tougher the challenge, so you can't have a fighter with 50 Strength getting the Intelligence options meant for the mages because he has 30 Intelligence in the late-game. Such bonuses would have to only regard the base skill, not any bonuses granted from items.
It would also be plausible to give money a weight. In many games, equipment has a weight that counts towards your carrying capacity, but you can carry limitless amounts of money. A single coin doesn't weigh much, of course, but it adds up. Assuming a gold coin weighs 5-10 g, 200 or 100 coins already weigh 1 kg, and a fortune of 100000 gold (500-1000 kg) could not be carried by anyone. A bank account might be a way out of this, though at a risk - the bank could be robbed, which might even be the start of a quest to get your money back. Collecting valuable gems and jewels could be another alternative: A diamond doesn't weigh much, but can be worth a small fortune in gold. Some traders might not accept gems as payment, however, and a few swindlers might try to sell you fake gems. If you've got some expertise in jewelcrafting/gemcutting, you should be able to see through their sham.
Money having a weight is an okay idea, as long as the most expensive items you need to buy don't require you to hire a cart and donkey to carry your money to the corner store. Gems as currency is another good way to mitigate the weight factor.
One thing though: I seriously cannot see a game that has a jewelcrafting/gemcutting skill, never mind one that's worth putting precious skill points into.
