There was mention of winning cars through some voucher system -- for prize-cars specific to whatever series you just participated in -- and that you can redeem for yourself or gift them to other players / friends.
If some of the prize-cars are treated anything like a handful of the ones in GT4, it stands to reason that prize-cars in GT5 that cannot be bought have to be either earned through race-series wins, or gifted from other players.
That's my understanding of it, though I could be completely off.
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