If you are talking of bying links from those sites that sell them then be sure that they won't list those links as sponsored oned because you can get your site penalized.
depends on where you buy it from. google doesn;t like to see people being in the practice of buying/selling links and wants to see the sites to grow naturally. but if you can buy links descretely and the selling site doesn;t show any indication of selling any links, then it would work out just fine
Google dont like it and it can consider a grey hat.... Many people are still use this way since this is the fastest way
i've always been told to steer away from the temptation of buying high quality links. Not only that but it is hard to monitor them all and make sure they are all still active links a few months down the line. it's a risky method which i have seen work for a few people, but it's whether or not you want to risk having your site penalized.
Unless you are willing to put the time and energy into learning a lot about link building and carefully monitoring your efforts on a daily basis, you run the risk of throwing away a lot of money at best and seeing site ranking penalties at worst.
you can hire somebody who has a great network of webmasters and can place your link on their websites. Better than explicitly buying the link from a link market ..