Can I sell art made with these tools?

Usually under the vendor's commercial terms, but ownership is a separate question from permission. In the United States, works without human authorship are not copyrightable, and the Copyright Office has held that purely prompt-generated images do not qualify, which means you may be permitted to sell something you cannot stop others from copying. Other jurisdictions differ. Check your plan's commercial clause and, for anything you need to protect, document the human editing involved.

How do I keep a character consistent across images?

Prompting alone will not do it. The features that work are seed control, reference-image conditioning, and training a small custom model on several views of your character. Expect variation regardless, and plan to fix the remainder by editing — consistency across a long sequence is still one of the hardest things to buy in this category.

Is it acceptable to prompt in a living artist's style?

Legally it sits in genuinely unsettled territory; reputationally it is a known flashpoint, and several vendors now block named living artists in prompts. Style itself is generally not protected by copyright, but the outputs can come close enough to specific works to create exposure, and commissioning contexts increasingly ask. Describing the qualities you want — palette, medium, era, composition — gets you most of the way there without the question.