If you buy a tuned car via memory-card trade, the parts are not purchased with it, even though you pay the cost for them. The parts are fitted, but can easily be lost if you change parts.
This is related to the "infinite supply of parts for all instances of the same model" feature. Once you buy a part, it is registered in a huge table as having been bought. If you buy or win another instance of the same car, those parts will be available. (Note that a prize car in this case is not the same as a similar model which you can purchase; each has its own garage code, and so a different set of parts). In fact, even if you sell all instances of that model, the parts will still remain available if you obtain another one later. (This feature does not apply to modifications which cannot be undone, such as weight reduce or engine polishing or balancing).
So you can see, the bug is that when you purchase a used car which has lots of removable parts fitted, your "inventory" of parts for that car is not updated.
I observed this problem in both North American versions.
It got fixed in [size=+1]GT2[/size]; in [size=+1]GT2[/size] you must buy separate parts for each car; no sharing! And when you buy a used car, you do pay for and get its parts.
In [size=+1]GT3[/size], however, when you purchase a used car, you don't pay for its parts, but do get them for free! That makes for some rather silly gameplay possibilities.
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