If you choose the 'Grey Market Connections' Andromeda perk, It is possible to abuse a bug with the merchant Thrasia at the wind farm on Kadara for unlimited credits. This would work with conceivably any merchant that offers you a personal discount, but it is a sure thing with this method. If you complete Thrasia's mission "A Package Deal" and return and demand a refund, she will refuse but offer a discount at her store. Normally this discount wouldn't break the merchant, but with the Grey Market perk, you will sell items for 10% more than what they're worth. This way you can sell her large amounts of rare materials at once, and then buy them all back and get a large chunk of credits for your trouble.