The good thing about PageRank is that it is blind. Unless something inappropriate is going on with an individual, Google does not care about the humans who own/buy/sell websites. They focus entirely on the domain name and the website behind it. Changes in ownership are inconsequential to Google.
Pagerank rises and falls entirely on the merits of the website and Goggle's algorithms. The risks of PageRank falling or rising is purely technical.
If anything, Google is very forgiving of websites that go inactive. There is a huge aftermarket of entrepreneurs seeking inactive domains with PageRank attached for its reaale value. Google often times is slow to identify inactive domains. This is mostly due to the fact that just because a PageRanked website goes inactive, it's links can remain active for years.
Last year I purchased an inactive domain name with a PageRank of 6 with several hundred links showing up in Google/Yahoo/MSN. Six months later, its PageRank dropped to a 4. It all honesty, it really should have been a 4. I just got to enjoy a PageRank of 6 for a few months. What this tells us is that Google is going to sooner or later correct a website's PageRank. But Google isn't the bad guy here, they are just applying the rules as strictly as they can with what they know. Google doesn't really worry about buying and selling because they know that the technical factors will keep everything even in the long run.
Purchasing a website in itself is no more risky than having owned it for years. A potential buyer needs to understand what the websites SEO strategy has been and how it gets its traffic. If the former owner spent a fortune purchasing traffic and the new buyer isn't planning on pursuing the same kind of traffic then it isn't unreasonable to expect the performance of the website to degrade. If the former owner was engaged in "black hat" or other inappropriate activites in creating traffic and search engine visibility and Google catches on, then the website will suffer along with whoever might own it. When purchasing anything of value, (including websites) the buyer must be diligent to protect themselves.
