Instead of buying links, you should start some genuine strategy to link building. Because purchased links are mainly spam and are not too good for seo purpose. If we create back-links manually, we can check the quality of the site. We also check website DA, PA and PR of that site. We mainly create backlinks to those sites which are related to our website. But in case of buying links, it is not possible. We get backlinks from random sites and hence they doesn't effect so much.