Buy a link - What to look for to avoid fraud

I need your help to make a list of factors to look at when you buying links.

- Site age (if site has PR6 and its 3 month old something suspicious)

- Check number of indexed pages in google

- SeoMoz (Need your opinion about that one)
Not sure about that one as I saw that some pages with good alexa ranking and good pr have very low or even 0 SeoMoz pagerank

- Some sites provide you checking tool which will check if PR real

- Alexa ranking ( pr8 site with alexa rank over 3 millions looks suspicious to me ) Need your opinion about that one

- Majestic SEO - will provide you at least number of backlinks, but again I saw some of the sites with huge number of backlinks and it has only pr1 or pr2 so not very reliable.

- keywordspy.com - will tell you how many organic keywords are in , usually high pr sites already have good serp, if keywordspy showing you only couple keywords look suspicious to me but again, Need your opinion about that one

Please share your factors and tell me if I am wrong in described above, I would be happy to see your ideas

If you're buying links directly to your money site you should always be careful. On the other hand buying a link and then pointing it to a buffer site is expensive.

I personally prefer to build Tiered links instead of buying direct links, it's just better value in my opinion and there is less of a chance that Google will burn you for it.