Pretty simple, just never sell anything. Since you won't be selling anything, money will come a bit slower, obviously, so make sure you're very deliberate about what you buy. Only buy weapons from Jimbo, since he's the only one with extremely powerful weapons worth buying. Other vendors do sell weapons that would be an upgrade, but other than Jimbo's weapons, you won't make it very far into the story before you find another, stronger item anyway, meaning you wasted your money. For clothing, again, items you find in the environment will be stronger anyway, so only buy those items which you need for quests. The reason for the stinginess is that the Hasselhoff plastic surgery, needed for the "Face Hoff" achievement, costs $175, so you want to be sure you have enough money for that.