What license does this pack have?
Can I modify the contents and use them commercially after buying them?
Yes, as long as it's part of a project and you're not re-selling the assets themselves.
Does this pack only work with RPGMaker or does it also work with other frameworks and engines such as Libgdx and Unity?
You can use it for anything. It's just the graphic files. Depending on your engine you might need to re-organize some of them, but you can use them in anything.