Another thing in several games is that you tend to gather a lot of powerful items throughout the game. If the game also only requires you to be really good with one weapon or armor type, you can simply keep the very best items and sell off all the others. Some games make it just a bit too easy to walk out of a dungeon with six Swords of Awesome, all worth a lot of gold.
Also, it sometimes happens that, at some point in the game, the need to buy anything apart from training is usually eliminated. By that point, you're pretty rich and the amount of money you have simply keeps growing, no matter how hard you try to spend it. To actually lose money, you'd have to start training all the skills that you didn't actually pick at the beginning, just to make a kind of uber character that can do anything.
I was writing this with Morrowind in the back of my head, but it's not the best example because it does actually have merchants who can't afford most of the really strong items. Problem is that almost everyone knows of the Creeper and Mudcrab merchants, and if you allow players the opportunity to get rich quickly, even in the form of an easter egg, they'll take it for sure.
Perhaps it would work better if you had merchants with limited gold, with actual legal restrictions on what you can sell, which have their consequences if you try to break them (in MW, they talk a lot about Dwemer artifacts being banned, but you can't actually get into trouble for trying to sell them), without easter egg merchants with lots of gold, and with some kind of living economy, rather than just having traders keeping the items you sell for all eternity, and combine all this with a living economy.