I leave the lid open which gets me a black background, I prefer that to the white background personally. Just a heads up, you’ll get better scans with the slabs out of those sleeves.
For selling - always a black background to maximize visibility on whitened edges. If you’re just scanning for yourself, then coordinating background colors or patterns to complement the card can look nice.
Agree with the fact that if selling use a black background no question, but overall I prefer a white background for my personal scans. It just has a sleeker feel to me.