Here's a great way to score infinite Credits. Follow the steps below in order to do so...

1.) First, fulfill the side quest involving Dr. Michel (from the Med Clinic) and her blackmailer. After the quest is complete, you'll get a discount from all of the goods she sells.

2.) Start selling her all of your expensive equipment. Then, travel to the Wards Markets and talk to the merchant named Expat. You can buy all of your stuff back for cheaper than you sold it off.

3.) Head back to Dr. Michel and sell off your stuff again. Repeat this process as much as you want for as much profit as you desire.