replies to your questions are pretty simple:
a.) NO. You definitely never know precisely what you are buying, no previous earnings, no pr, no backlinks, no traffic does guarantee you, your site will be successfull in the future!
b.)you can minimize your risk by multiple steps.
Always check for: - traffic (request proof -extreme tracker or analytics is the best)
- revenue (request proof)
- age (not domain, but website)
- source of traffic (!!! very important - can tell you, if the traffic is natural, from SE and will stay, or is from paid links, which can be dropped)
- search engines position
- and many many more aspects
hope this helps you a little bit 
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