There needs to be a note on top mentioning the cap. Maybe, "Gran Turismo Marketplace (Trades limited to cars worth less than $1 mil.)
Would save a lot of confusion. Not all. But a lot.
There is a Sticky at GT Planet's Marketplace that lists all untraaaadable/blacklisted cars in the entire game. Every single day, many people skip right by it and offer to trade untradable cars.
Moral of this story: Reading Is Fundamental. Most people seem to think that reading is for everyone else but them...........