Well, some well known games that have had this problem in my own experience: Baldur's Gate 1 & 2, Icewind Dale 1 & 2, Neverwinter Nights 1 & 2, Oblivion, Skyrim, Fallout 1, Fallout 3 (not sure about fallout 2 - I seem to recall it wasn't such an issue?), Mass Effect 1 & 2 (3 had some interesting ideas in this regard).
You could make money scarce - and I believe that it a part of the solution - and you could make high barter skills necessary but then the issue you would face is that not all players would focus on barter skills and so would need more money. But money is scarce so you would need to make purchasable items not necessary so that players who don't barter aren't at an unfair disadvantage. But if purchasable items aren't important, then why buy them? Then you've got the problem of what to spend your money on - so you end up stockpiling it and hey presto, rich as sin again.
I think you're going to have money, you need viable money sinks. As I understand it, that was sort of the idea behind health and mana potions in the first place but it was crude, not very effective but never really got developed.
@Thraxen, you and I appear to be in accord I think. Rewards for investing money should not be more money - just compounding the issue. It should not be better equipment either, making fights easier as, if fights aren't balanced for that then fighting is too easy and boring, so you are punished not rewarded, or if fights ARE balanced, then if you don't have the funds then you are stuck.
It should be something tangible for the player. So, new content seems like a good alternative. I like your expedition idea. Similar to Dragon Age's dwarf plot but in a far more involving and practical way. The more things you invest in could alter the course of the expedition