I feel that it should be via the PHB rules as then it will create a new economy and breed issues See below for example (V.V. I'll use your wizard as an example)
So VV's wizard has robe of contingency +1 he'd like to sell.
Player A & B want it as the item represents a 75% savings (or even 50%) over what they'd pay for it. If A gets it B's upset, or vice versa, as now they have to buy it at full face value and are out x gold pieces over A.
Also if 50% is the new sale price, gives more back to initial player, thereby messing with the economy of the game. Also, judging will be much more difficult as you have to now set up a whole magic item marketplace, tracking who has what, who sold it to whom, etc, etc.
From a conceptual POV I like the idea because it makes it more like a Living setting, but at the same time I can't find that benefit to be more than the others I listed above.