I was thinking a little about this issue, too.
I think this boils down to three things:
- Merchants are not warriors
- Two pieces of good equipment trump one piece of better equipment
- Why aren’t merchants getting robbed?
To elaborate 1): merchant and warriors (adventurers, heroes, whatever) are two very different professions. If a merchant was to use this powerful relic for themselves, they’d have to (a) know how to use it, which might require prior arduous training – this legendary sword requires 150 str, while an average merchant has only 20 str; and (b) be willing to accept the risks inherent to fighting (OK, so I have this powerful magic orb, and I can now go wild and start killing people… but wait, what if I’m countered by two other guys with a similar magic orb each… or what if I’m countered by a guy with an even more powerful magic orb… or what if some local kings dislike my killing everyone and my wanting to become a warlord, after all there should be several such magic orbs in the treasury of each kingdom… I think it’d really be better to sell this orb to this guy who’s just came to my store, let him take the risks, I’ll focus on making profit from trading)
To elaborate 2): Well, the price for such artifacts should be, in my opinion, set appropriately high. Not necessarily sky-high, though: let’s say that a few pieces of equipment that is a grade lower in quality can make up one piece of equipment that is a grade higher in quality. The merchant can still make profit from such a deal. The reason is that a gang/party/squad armed with a few pieces of a little weaker weaponry is still likely to defeat a gang/party/squad armed with only one piece of a little stronger weaponry. Three guys with a +150 mithril sword should still be able to kill one guy with +200 adamant sword accompanied with two guys with a +10 wooden training sword each. Overlords/warlords/anyone else who may want to arm his men for any reason that might involve fighting should be aware of this.
To elaborate 3): This is, in my opinion, an even better question. Suppose you see a lone merchant with stock full of precious artifacts in the middle of wilderness. A thug/bandit/servant of an evil empire would have no restraints to simply kill the merchant and loot his shop. Even heroes, who are supposed to be more moral, might think twice if their very survival depends on getting this particular artifact. There is, of course, the argument that long-term it wouldn’t be beneficial for anyone, since this way there would soon be no merchants to trade with; however, this argument is weak, since of course immediate survival trumps any future potential profits.
To fix 3): I think this issue should be fixed if we stick to this one rule: All merchants must have somebody to enjoy protection from. For example, it would be perfectly fine for a merchant to operate on a marketplace in the capital city of a powerful kingdom, enjoying protection from the king’s elite guards in return for taxation. In such a case, however, the king is likely to want the merchant to sell the most powerful artifacts only for the state, definitely not for random strangers. This, however, raises no problems if the player is himself an elite paladin of this kingdom. Similarly, the enemies, say the orcs for example, might have their own merchants to trade with, who routinely travel to orc-occupied lands to sell them goods. Such merchants, however, might be unwilling to trade with enemies of the orcs, for example the players, or else they might be beheaded by orcs. This may change if the orcs start suffering grievous losses from the player, for example when the player captures the marketplace the merchant was previously trading with orcs on, drives the orcs off all nearby lands, and by some miracle the merchant and his goods survive the battle. Another possibility is that for reasons the game plot must’ve previously explained the merchant wishes to sell his best wares particularly to the player. Chances are he might want to do this in secret, though.
I’d suppose this is how this setting could survive base logic.