Good points, if I have a choice to put one link on other site then I should put homepage or category or post URL on their page? what is more benefits?
For example, if i buy a link on a high quality site, i should put my homepage URL or targeted category on?
It really depends on the situation. The link should point to the most appropriate page for that link. That might be an inner page or that might be your homepage.
In general, I think you are much better off focusing on getting links for the inner pages of your website where the best content is on your site.
If an investigative reporter is writing a research article and is explaining something and then links to a website as a reference, he is going to link to the most appropriate place that helps the reader. If he is linking to a website that has a deep explanation of something complicated, he is probably going to be linking to an inner page that contains the explanation of that complicated topic. If he is mentioning a person by name and saying that person runs XYZ website then he is probably going to link to the XYZ company website homepage.
If the link is being used as a reference for more information then the link is probably going to point to an inner page on your website. Just like the links at the bottom of a Wikipedia page. If the link is coming from a list of websites that sell product "X" then that list of websites is probably going to link to each website's homepage.
Do you get what I mean? The link should point to the most appropriate location on your site. Sometimes people will try too hard to get links for a specific page on their website. Don't waste your time and energy doing that. Get a link to any page on your website, especially the inner pages. And sometimes people focus too much on trying to get a certain anchor text in a link to a certain page. Don't waste your time on that. It isn't worth the extra energy of worrying about that. Just get the link to whatever page you can get it. If you can get the anchor text you want, then great get it. If you can't, or if it is going to be a pain in the neck to try to get it, then don't worry about it. Just get the link to any page you can with any anchor text that you can.
Do you really want to start buying links though? Do you think that is the best long term plan for your situation?
If you are just thinking "I need to go buy links and I just need to figure out where to buy them from and where to point them to" then you are on track to getting your website penalized and demoted in the search results.
I challenge you to think about it differently. Think about this: What kinds of sites in my industry do I consider to be high quality sites right now? Why do people tell each other about those sites? Why do people link to those sites? What types of other websites link out to those sites? What would I have to do to get those sites to link to my site too? What would I have to do to get people to tell each other about my site? What would I have to do to get people to talk about my site at all?
Asking yourself those questions is a really beneficial exercise that will point you in the right direction for promoting your website.