I've personally used services like this one and others just to test the efficiency of it vs natural link building techniques and the conclusion I came to is that even if it was legal, it would still be a waste of time. These types of bought links fall away eventually as they make space for the next person wanting to buy it and I think that a certain aspect of those sites are focused on automated / generated content which will get flagged later on turning those high DA links into spammy links that bring over penalties and spam score anyways. The links you can buy that are "safe" would be costing you around $400 per link so unless if you have a huge piggy bank ~ that's not an option either. Advice would be to forget about paid links completely and focus on the guidlines the search engines give you and just go the natural route.