Some observations on the topic I gathered over the years (mainly focused on the old infinity engine games):
- In most games, shopkeepers will be able to instantly come up with any amount of cash to purchase items from you. I recently sold a single magical flute in lowly Kuldahar for more than the likely value of the whole town. Limiting the amount vendors can spend in a certain timeframe will certainly not eliminate the problem, but at least slow it down. At least it took me longer to get rich in the elder scrolls games.
- As stated above, there are almost never items on sale that are better, or can even compete with the stuff you take from fallen foes (especially the kind of foe with unique name and some dialogue before you eviscerate him). Kinda odd how their old and used junk is better than anything crafted in the realm.
- Non-magic equipment is sold way below production cost. A suit of leather armor is cheaper than a drink in Easthaven.
- As already mentioned, potions are useless after the early game. 9 hp restored out of 150? Strength set to less than what I'd have without? No deal.
- After some point every half decent enemy will wield a valuable magical weapon, driving item inflation to new heights. You can probably blame the old "Protection from normal weapons" effect for that one. Might be a better idea to make foes tough via the skill with which they use their equipment, not the gear itself.
- Slowly degrading equipment might make a decent money sink. The key element here being slowly, not those single use swords from the beginning of baldurs gate.
- Since all of this makes you rich rather fast, thievery is generally quite pointless. Though this is probably another topic altogether.