The idea is good - the sale price thing isn't. It's completely counter to common sense that common items have the highest disparity between selling and buying price. The more common the item, the less the disparity between buying and selling an item should be.
Yes, I agree, but the problem is more serious. It actually just doesn't make sense that the sale price is 20% for an item that does not deteriorate in value!
True enough, if I can only sell an item for 20% of what I originally paid, then the item is very common. However, if an item is common, then I would not have had to pay 100% for it in the first place!
An example: my character wants to buy a +1 sword, and goes to the local merchant who is asking 360 gp. Finding this price too high, my character stands outside the merchant's shop with a sign saying "I buy +1 sword for 100 gp!". Since +1 swords are common and everyone who tries to sell only gets 72 gp, I will soon find a seller at that price!!
The only thing the merchant is offering me, is a wider choice of items, and the fact that I can save myself some time and effort in acquiring it myself. But this does not justify a 400% markup!
Realistic would be that the buying and selling prices are fairly close together for both common, uncommon and rare items, because the items do not deteriorate in value. And, as you say, the bigger difference in buy and sell prices is most likely to be for the rare items (and the reason for this is because the merchant has access to more buyers than I do).
Basically, the huge difference between buying and selling price is economic nonsense, and this has not been corrected by the new rules.
But, we probably have to accept this strange system because it just makes sense from a game mechanics point of view, for a number of reasons:
- It provides a way for the D&D economy to "burn" cash.
- It makes you more careful about the items you choose to buy (make a mistake, and you loose cash!).
- And it also makes items that you find "more valuable" than those that you can buy (because you can't just sell them to buy what you really want!)