I don't like the merchant skill idea. Also, I think the reasoning here is wrong. If you increase general upkeep, it will be the poor people who suffer more than those who have the huge stockpiles of gold. This suggestion may conceivably solve that issue in the long run, but short of a wipe, I wouldn't count on it.
That said, I do think upkeep in general should be higher. People whine and groan every time upkeep is raised, because as you said, they're spoiled. Nobody likes to have to downgrade, even if it would end up making the game better. Many go as far as to take it personally, as if they're the only ones with higher upkeep, saying things like "OMG now that I have to wear this crappy shit, I can't kill anyone and everyone kills me in one hit!" This reasoning completely ignores the concept of relative strength and the fact that if everyone is forced to use crappier gear than this won't be the case at all.
As it stands, the majority of items, weapons especially, are incredibly underrepresented. The only people who use anything other than the highest tier weapons are people who are just starting out. Otherwise, who uses a light one handed battle axe, when they can easily use a broad one handed battle axe? Not even including valour, the average player (wins 50%) can maintain 46k gold worth of equipment, not including arrows or low wpf weapons. This could be a *lot* lower.
At the moment, the only real economic decisions you have to make is how long you can run around in full plate or armored horses. I think the game would be much better with more meaningful trade offs and opportunity costs. If you want higher tier armor or weapons, you'll have to make a sacrifice elsewhere in your equipment choices or in your wallet. As it stands, we have far too many knights.
Of course, you wouldn't be able to increase upkeep in one fell blow, without inspiring outrage and indignation, so I imagine it will have to be gradual. I liked the strategy (at least that's my theory) employed this last patch. Raise upkeep to by some large amount, all hell breaks loose in the community, then readjust upkeep such that it has still increased from prepatch, but now looks much more reasonable compared to the upkeep prehotfix. This is a standard technique employed by parents of spoiled children.