Realistic textures or hand painted textures?

If you're just selling textures, then realistic PBR textures are probably the top sellers but there's definitely room for NPR textures or materials as well, especially for game-ready models.

Photorealism is the most common style in 3D art so there is a lot of demand for realistic textures. But there is a lot of competition too, a big portion is even free. A unique style is a lot more niche but might get people to buy many of your textures in that style.

People are more likely to buy something that is better done. Different styles are needed and important, it all depends on the specific needs.