Purchasing a copy of a base pack from me grants the user personal & commercial use of it. This sale is the grant of a license, not a transfer of title. When you purchase a copy, you are licensed to use it for both Commercial (for-profit) and Personal usage such as:
• Character designs & adoptables, merchandise, YCHs, fill-ins, ref sheet commissions, etc.
As long as you are coloring in or otherwise using the files as a building block/base to make a new unique piece of art, you may sell the resulting art! The only thing you may NOT do is remove or refuse to give proper credit, or transfer the files in a transparent colorable format (.PNG)