Pretty much, the secret to RAM buying is this:
If you want 8GB total, buy a 2x 4GB set.
If you want 16GB total, buy a 2x 8GB set.
This is for a dual channel motherboard, obviously. For a quad channel board, if you wanted 32GB, you would buy a 4x 8GB set.
You get the idea. The old set, can be saved for emergencies, or, like you said, sold on a website.