Many websites benefit from the purchase of links. However you should not do it. This is a grey area to say the least. Matt Cutts at Google has come out unequivocally to say that sites that buy and sell links will be penalised. However it’s a fact that many household name websites purchase links. In addition, paid for directories that include a general review or sites where membership confers other benefits are acceptable if they charge for an entry on their site.