I think a few points have been overlooked here, and I have something of a solution ( though not perfect, it's workable. )
The whole 'it's a racing game not a trading game' argument is laughable. Trading is part of the social interaction put into this GT to allow players another way to meet new people. It provides a break from grinding and affords people the chance to trade with each other for cars/paints that may rarely come up in their GT life. I'm fine with one per day, but value limitations have killed this for me as I've over 480 cars now, and most of the cars I seek rarely come up in my UCD, but may do in my friends.
I feel if PD imposed a trade value dependent on your game progress it would solve the million limit issue. For example, someone at levels 0-20 cannot trade in cars over Cr. 1,000,000, but those at levels 20-30 can do so up to 2,000,000. With a cap at levels 30-40 being around 5,000,000. It has a lot of benefits with cross level trading too. If a higher ranked player needs a rare car from a lower ranked player and swaps an X2010 for it, the lower ranked player cannot go on to trade that car till he ranks up. It also means secondary accounts, being usually starter levels, can't trade back for duping.
It goes deeper than just that but I'm harping in a bit. Let me know what you think...