First of all when buying links, make yourself familiar with the standard market prices of the niche you are looking for. The niche of the website you want to buy links from should better be related to your landing website’s niche, as it will directly affect link quality. Check dp forums and find out some nice offers and compare prices.
First of all, I recommend you downloading seoQuake, as it spared me lots of time when surfing around.
1st Rule: The Less OBL (outgoing links) the website seller has, the better.
2nd Rule: Check domain age so you don’t buy a link from a PR6 drop domain.
3rd Rule: The more backlinks the website has, the better.
4th Rule: Don’t buy links from websites with different niches. The website should contain content only on the specific niche.
5th Rule: Always negotiate every details, including refund possibility.
6th Rule: Never accept first offer.
Good Luck, daniel.