Some time ago put my hands on a used copy of Gran Turismo 3, the only title I hadn't play from whole series.
Stating that is slow to earn money in GT7 makes me laugh, when start racing the Sunday Cup to earn 1000 credits for win, where, by comparison, a '99 Mazda Demio was more expensive (14660 cr) than the '15 Mazda Demio that you must own to complete the first menu and you would only need to finish top 3 in a race which you only need to turn left literally. You would only win a car after winning the 3 races of Sunday Cup.
So, in GT3 you buy a car (have to change oil if you opt for a used one) win three races to end up with 3k credits and a prize car worth about 5000 cr.
In GT7 you buy a car, win two races, end up with 10k credits (15k if you achieve clean race, which you even can it walls or exit track, just not crash into another car) and another 2 prize cars with a combined value of about 27k.
The most you could make from a race on begineers level was "Gran Turismo World Championship", where you would race rally and GT300 cars, to a prize of 10k a race, against cars costing 750k.
Personally, without any grind, I'm on menu 25 and golded licences until International B. Only bought the Toyota Aqua S initially being mandatory, since than, have a 39 car garage without spending another credit apart of the 14.3k spent in the Toyota. Spent about 600k in tuning some of the prize cars to compete and advance. Have about 1 mil credits available.
After a couple of hours in GT7 you can grind in High Speed Ring with a prize car (Supra Gr. 4) to almost a million credits / hour.
About the used cars being only temporary available, it's a stalwart on another GT games too. Who can forget the black cars in GT4, who only appeared in 700 days (each race 1 day) cycles, starting in days 694-700, with the whole used car lot refreshing every 7 days?
