What if I downloaded my tracks legally, instead of purchasing the actual CD, can I still sell my used digital copy?

When you sell a used CD, you are not making a copy of it. You are selling a hard physical thing and you no longer have it. If you were to sell a used digital copy, you would be making a copy of it. There is no other way to transfer a digital copy than to transfer a copy of it to someone else. Since you are making a copy, that violates Copyright law, which gives the right to copy to the owner of the copyright on the sound recording. Therefore, selling a digital copy of a song you bought legally is illegal.