Now, dusting off my memories of my undergraduate courses in psychology and game theory, I think the key issue is not so much one of "realism" as it is of "fairness".
After all, there is nothing particularly "realistic" about the 50% initial selling price, 100% resale price model that we have gotten used to as the standard, either. It's just that a 50-50 split between the principal (the PC) and the agent (the merchant) seems somehow fairer as opposed to a 1:6 split, considering the initial selling price of 20% and the potential resale price of 140%.
As a DM, you could certainly allow PCs to buy and sell magic items at 100% of the standard item value if you want (for the record, I pretty much did this in 3e). Just be prepared for the players to optimize their PCs' items (unless you give them a restricted list of items they can purchase) and for the resultant knock-on effects on gameplay.