How do I make sure my online seller is legit?

As someone interested in cryptocurrency and finance, I understand the importance of ensuring the legitimacy of online sellers when engaging in transactions. So, how can I make sure that the online seller I'm dealing with is legitimate? What steps should I take to verify their identity and reputation? Are there any specific tools or resources I can use to conduct thorough background checks? And what are some warning signs that I should be aware of when dealing with online sellers?