If you find yourself in need of a few hundred thousand credits and can’t be bothered to grind, take all the shells out of expired rental tanks and sell them. I did this with the expired rentals in my garage and got around 300k credits.

Also, since rentals usually come with a 100% crew you can throw crew books at them in the rental tank and then retrain the crew for whatever line you’re grinding. I never remove rentals from my garage for these reasons.