DM_Blake said:
Now you're on to a very good point.
Depreciation.
...
Please, right now, go outside and try to sell your 1969 Ford Mustang Boss 429 for it's full new price... For those not into cars, that car is now 39 years old, originally sold for under $5,000 (US) in 1969, and now can sell easily for over $100,000 (US) to the right collector, or for at least half that amount instantly at any car dealership in the US - still 10x what it originally cost.
If you're going to concern yourself with depreciation, you might also want to concern yourself with appreciation, too.
That's not "appreciation," it's (at least partially) inflation and rarity. Let's tackle inflation first.
If your 1969 Boss 429 has been fully restored, it might as well be new. So, the start of your value comparison is not what the car cost then, but what a comparable car would cost now. For comparison, a 2008 Mustang Cobra (not quite comparable, but close) retails for about $40,000 (US).
Now, we get to rarity. Is 2.5x a fair increase for something that is VERY rare. Of course it is. Yes, there are collectors willing to pay full price to (or through) a reputable dealer.
People are constantly trying to apply modern standards. Remember. No internet. No newspaper. No mass media of any kind. No telephone. A BIG city has 25,000 people in it. The total population of a region the size of the United States is probably 6 million.
You have something that maybe 1 person in 1,000 can afford. That means in the biggest city in the continent (a fantastical Waterdeep-sized metropolis of 500,000), there are maybe 500 potential buyers. In the average big city, you'll be lucky to find 25.
How many of them do you think an errant vagabond just arrived in that city is going to be able to meet in one day? One week?
Your merchant has the connections, but he's taking a risk. Is he getting an undue profit? Maybe. Maybe it should be closer to 50% of sale price.
However, at this point, I'm going to put forward a comment. If you're spending a lot of your game time on profession skills, craft skills, and trying to map out a realistic economy of the D&D world, your game probably isn't as fun as it should be.
My two cents.