I agree with all the methods people have described, but most require a high level character or a decent amount of seed credits to turn a reasonable profit. I personally Use a mix of these methods to net a good profit and mix things up so I don't get bored grinding for resources or running Heroics. If you are just starting out and don't have a 60+ character or a couple million credits to invest, however, the best way I've found to get credits is through Artifice crafting and doing the Heroics. The items you can sell for decent profit (lightsaber crystals, dyes, artifacts) require a relatively low level to craft and the resources are easy to find in the early planets. I've gotten returns of up to 500% or more depending on the GNT sell price.