Do I need to trademark a website?

LOCATION: Pennsylvania, USA

I am in the process of purchasing a website from someone else. It is a niche social media site of which I've been a user for a few years. The current owners approached me because they want to retire and I'm an experienced software engineer.

Yesterday I found out that someone else had made a relatively similar site (slightly different approach and purpose, but similar niche) with exactly the same domain name, but a different extension. The site I'm buying has been around for a few years. This other one is a few months old.

I don't want to run into legal issues in the future. What is the best way to make sure of that?

Thanks in advance for any advice.