Is it illegal to sell below cost?

Could you please clarify for me, is it considered illegal to sell a product or service below its cost? I'm particularly interested in understanding the legal implications of such a practice, especially in the context of cryptocurrency and finance. Is there a specific law or regulation that prohibits selling below cost, or does it depend on the jurisdiction? Additionally, are there any potential consequences for engaging in such a practice, and how might it impact the overall market dynamics? Thank you for your insights.