I just figured out an interesting strategy for grinding small amounts of money fairly easily.
Required - A few thousand credits (maybe about 8k, I'm not 100% sure, initial patience.)
1. Purchase a Daihaitsu Midget II D-Type from a used car showroom (forgot which one, but it's a fairly recent model.) This takes the "initial patience."
2. Upgrade to the first NA Tuning upgrade, getting some extra power at a relatively low price.
3. Enter the Daihaitsu Midget II one-make race. With the NA upgrade, I've won first place in B-spec. Haven't tried the whole race in A-spec yet. This is a 4 lap race on Motorland, and only one race.
4. The first-place prize is 2,000 credits, and the prize car is a 1963 Daihaitsu Midget. A puny car, even compared to the Midget II, but it sells for 4,000 credits or so.
5. Clear your record for the Midget II race and repeat.
If you're stuck unable to purchase the NA tuning, then enter a driving mission. As long as you have your IB license, you can do the "pass" missions. The Midget II race itself is license-less.