Circuit experiences are a very good source of money. You don't need to get gold on all of them but bronze is a must because they are easily doable. Best method to grind money is Tokyo expressway that pays out 550k normally and 825k for clean race bonus and it takes 27.5mins to finish. I got the clean race bonus at least 90% of the time and used Ferrari F12 with sport hard tire compound. There are also 2 races available for GT7 25th anniversary which payout 3.6 millions available for ~35days.
Selling cars could be useful for selling duplicates, 10 cars you bought for owning 10 of the same car, cars you bought for trophies (i.e., Audi R18 TDI, Dodge SRT Tomahawk X VGT, and BMW Gr.3) which could add up to ~2-3 million probably but nothing substantial.
