Pawn shops have no use for anything they buy except to resell and make money off them. In a dangerous world full of monsters is there demand for literally magic items that can improve your odds of you and your family not dying to those monsters? Uh... yes. Yes there is. The demand for magic items would be so mind boggling it would make Apple jealous. Look at it the other way, if demand was as low as you say, PCs would have no problem buying that useless junk for cheap.
Pawn shops only buy things they think they can sell. Watch Pawn Stars for a season; you'll see plenty of things that are valuable that they don't buy because the asking price is far too high to make a profit buying it.
How much is a +1 sword actually worth to an NPC? Using 3.5 era math for an example, 2,315 gp will get you a +1 longsword that nets you a +5% better chance to hit and a 12% more damage per blow. OR, you could buy the services of 30 mercenaries for a year. Which is going to protect your kingdom better?
Well, how about 30 mercenaries buying their own +1 swords? Well, at a standard pay rate of 2 sp per day, it'd take them 31 years of soldiering to buy said sword, assuming he never had to buy any other good or service ever.
Would a noble afford a +1 sword? Sure. He could keep one for personal protection (an heirloom) but he's not going to want part with it unless he's hard up for cash or can get a replacement one. Further, how many nobles are there in a given area that can afford +1 swords? Not enough to mass produce them (and mass production per RAW gives no advantage to creating in bulk, economy of scale doesn't factor here). So a noble might have one, but that is no different that an art market; small, insular, and primarily used as status symbols than the backbone of an economy.
All these things point to market controlled not by fixed price points, but by barter and haggling based on immaterial factors (status, history, rarity, need, or usefulness) vs. a fixed price for a manufactured good. This is not Apple selling an iPhone, this is Christie's auctioning off a Rembrandt.
Which brings me around to the point; an adventurer wandering into town hears the local lord possesses a magical sword. He walks up to the lord and says "I want to buy your magical sword. How much?" The lord shouldn't reply "2,315 gp, market value.", he should say "How much do you got?"