What license have this plugin. Is it MIT? How can I use it? Can I modify it for own usage in any way?

On a per-site basis you can modify PB as needed. If you find you need features that are not there, please feel free to communicate changes you make that we might consider adding to the app. Or you can package your changes as a plugin in and of themselves for sale that "extend" ProBlog as a required dependency. (see the example "extending the user plugin" tutorial)

Thanks for answer.
Can I use PB on more than on site or I need to buy separate license for each single site?

It is a per-site license.

But after purchase. Can I fork it for own usage and develop only for my own? In Joomla for example all products in Joomla Extension Directory must be GPL. So I can use it for my own reasons. I don't want to resell your code to anyone as a separate product, but use it as a base to my own plugins. Some parts of it, or whole, as a starting base. To better learn best practices in october, and have solid base to own work.

So, how it is with your code? Marketplace terms says :

When making a purchase on the OctoberCMS Marketplace you are obtaining a license to use a theme or plug-in, you are not becoming the owner of that item. The license and its terms are exclusively from the author, October CMS does not set the terms of the license agreements or the type of licenses available.

So it's up to you, if you allow me to reuse this code in my own work or not. I want to know. Because sometimes it great for me to stay with original product updates for projects and in many cases it will be like this. But sometimes I want more freedom... :)