V.V.,
Unfortunately, what you propose still causes an issue in the DnD economy. If I buy something for 1/2 price from a PC, but my friend that wants the same item as me, is the same level as me, and has the same amount of gold as me wants that item too, he has to buy it from an NPC for 2 times the price that I paid for it. Now if this happens only once, no big deal. If this happens multiple times (and I'm sure we would see this happen) there could be a significant difference in power level down the line, as I would have more gold to buy better items down the line due to all the money I saved. This would also be true for he/she who sells their items for 50% rather than for 1/5th (20%). Eventually they would end up with more gold than they should for their level which would allow them to buy better items, causing another disturbance in the power curve. All in all, I just think its a bad idea because it means that SOME people will get their items for cheap, while other will have to buy them at full price. Some people will sell their items for 50%, and other will either have to sell them for 20% (1/5th price) OR they will hold onto that item until they find a buyer, which may never come about. Either way, they are down on gp in comparison to the PC who sold to another PC. On top of this, even if we place a "only once per item" limit on PC to PC sales, that means that whoever bought the item from another PC would get to sell it for 1/5th the price once he is done with it. All in all, that means that (after the final sale of the item to an NPC) the user has only spent 30% of the base price of the item (save 50% on initial sale, then gain 20% back when selling to NPC = 70%). That is a big power flux down the line if it happens multiple times.
