I have a start-up idea, but I noticed that a similar service/website/database/business model already exists. It operates in a different niche/targets different customers, which is good. However, it is far more mature than my concept and it would be unwise not to learn from how they have grown and what they have tweaked/added. I'm thinking that this inspiration may lead to too many similarities. What should I check and consider to avoid intellectual property infringement and other legal issues?
Firstly investigate if there is anything trademarked or patentable (I doubt the latter due to court cases like Alice killing off a huge amount of business process patents).
Just don't do anything completely stupid like steal any copy from the site and repost it.
The fact is that there is no end to situations similar to yours. Just because someone is there first, doesn't mean you can't do the same thing - even in most cases exactly the same thing.
In fact, you're in a better position bearing no legacy and not having to pay the heavy price of educating the market on the value of the offering. You just have to innovate within it, or market better, or expand the market.
If you're emulating someone else's business model, technology, or style, then once the time comes to choose a brand name / domain for your own service be sure NOT to imitate theirs.
That only covers 1 aspect of IP, of course.
You can go to the US Patent and Trademark search engine or Google Patents and do a search on the company name. This will pull up any filings assuming its under that company name.
This is where its important to also obtain the names of other key figures who might have filed the intellectual property under their own names, rather than that of the company. Perform additional searches using these names as well.
A good IP attorney or agent would be worth the money to pay to have them conduct the search.
But keep in mind that functionality, like that of the Amazon "1-step" purchase button, gets a lot of attention, but there is little that you will see on the internet in terms of platforms that are novel.
Hope that helps! Bets of luck to you.