Some link builders obsess on buying links, linking from other sites to another.
Without knowing the value, assurance and sites background.
Here are the guides that needs to watch out :
1.) Referring root domains
- Linking from a website with a high referring pages than root domains.
2.) IP address
-Website with different site and links structures. But has the same IP. Which means the same owner and probably same hosting
3.) Auto generated pages
- Pages with an extension. Examples :
4.) Crappy sites and not your NICHE (kind or industry of the site)
-Website that links to a junk website
-Not related to your site/Niche
5.) External Links is more than 30+
- I wouldn't say 30+ external links is not worthy. But passing the above mentioned, external links would be my last option. And was based my experience and the fact that matt cutts said no more than 100 external links.