From my experience, the BST section is a total crap shoot, and finding a good seller sometimes means sacrificing a site or two to crappy sellers. If you are doing this for a client, you may be in over your head unless you already have an outsourced system down pat. If your already on the clock, I doubt you have the time to trial run some of these, SEO is a patient mans game. However, as a rule you should purchase from sellers who are active in the community and have a lot invested in their brand/name. A couple of other tips that helped me, was to click on some of the more seasoned SEO guys and look at what they are purchasing, and read the threads. Also, if these same guys are coming back for 2nds, 3rds, and 4ths, you may have a winner!
There are only three types of links you should buy.
1. Natural link diversification/anchor text proliferation: Lots of authoritative links/niche relevant links(Press releases, top 30 bookmarks, relevant niche specific blog comments, video, audio etc.). You can normally find these very cheap, and they don't have to be contextual. Top niche specific directory links CAN work but they can also be poison too if you do them wrong, so stay away for now. All of these links can be nofollow or d0follow, and anchor text should be naked url/generic/brand.
2. Link Juice Links: (Web 2.0/Guest Posts/PBN). These are more costly and riskier, but they are the most powerful. The anchor text in these should be contextual, relevant, d0follow, and from the most juicy/authoritative sites that you can find. Unless these include homepage links from a powerful PBN, they should also be tiered out to boost up their ranking power, but that should be done smartly as well.
3. Tier 2 Links: (Everything). You want to get a mix of link types 1 and 2 for this using the same rules and spread them out over a month or two(in fact you should be drip feeding everything, and the most you want is to average about 100-500 links per tier 1 link over a month). My personal strategy is to purchase a package of web 2.0's, article links, bookmarks, and blog comments, all with generic/naked urls. I then blast the web 2.0's with cheap GSA link services found here. After a week, I'll purchase from a seller who sells tons of cheap PBN links, and I will use straight LSI anchors for these.
Purchase an expensive consultation service if you still do not know how to go about purchasing links. Good Luck!
