I think there's a danger here, which is to assume that the market for magic items is large enough to be treated like an abstract macroeconomic system.
Selling magic items? Sure, you can sell 'em, and probably for a pretty decent price. Every petty warlord and two-bit bandit chief would love to get his/her hands on a magic sword, and would pony up quite a bit of plunder for it. However, those same warlords and bandits would much prefer to kill you and take the sword for free; unless you're willing to settle for a very low price, the process of finding a buyer is fraught with peril.
(And this assumes that the magic items themselves are okay with being sold. That's not necessarily a safe assumption. Magic items are notorious for having strong opinions on who should own and use them, and punishing unworthy masters... which would be another obstacle to finding a buyer, though hardly insurmountable; there will always be folks with more money than sense.)
Buying magic items? This is where I think macroeconomic abstractions are dangerous. I prefer settings where non-consumable magic items are rare and often invested with great significance; they tend to be heirlooms passed down for generations, and the owners won't part with them for any sum of money. Creating a new magic item requires extraordinary skills and exotic, one-of-a-kind components--you can't just build a magic item factory to increase the supply.
It's worth noting that in every pre-3E group I played in, nobody ever sold magic items... not because the DM wouldn't let you, but because it wasn't a worthwhile proposition. Gold was easy come, easy go. A magic item was well-nigh irreplaceable.
As regards the comparison to research scientists/mercenaries/whatever in the modern world, I don't think it's a valid comparison, mainly because D&D typically assumes a medieval or pseudo-medieval setting. The kind of money that gets spent on mercenary gear and research facilities is the product of a society that generates a massive surplus of goods and services, orders of magnitude greater than any medieval society ever produced. If your fantasy society produces surpluses on that scale, it's going to result in drastic changes to the traditional setting, and magic item prices will be the least of it.
(Edited to add: I do grant that even in medieval times, comparatively vast sums were spent to outfit a small cadre of elite warriors. But D&D item prices scale far beyond the level of medieval gear. A 14th-century knight's outfit, including armor, horses, and weapons, might run about 20 pounds sterling; the same source puts a gallon of ale at 1 penny. A pitcher of ale in 4E costs 2 sp, and there were 240 pence to the pound, so we're looking at about 960 gp to outfit a knight. But high-level magic items are priced in the millions, enough to equip a whole army of knights for the cost of a single sword.)