Game economics are not real world economics, so don't even go there.
If credits are the goal, you have options. You can gather mats for free (because time doesn't give you credits for nothing) and sell them, or turn them into items and sell them.
If you want an item. You have options. You can gather the mats for free (again, because you aren't paying anything because time in game does NOT have an intrinsic value) and sell them for credits and buy the item, or you can gather the mats for free and craft the item yourself, or you can buy the mats and craft the item yourself.
If you gather your own mats and make an item, that item has cost you time, but not credits (unless you gather through missions). No credits have left your account. It doesn't matter what the potential value of those materials is since that value is unrealized until someone purchases them for credits and thereby set their value.
Real world economics and game economics, while similar, do not equate the same when it comes to values. Again, if you are buying mats to make an item, you are doing it wrong.
Here we have someone who doesn't get opportunity costs. This is not good advice.....It's just way too simplistic. This poster displays a gross misunderstanding of opportunity costs.