That's a tough one.  As you probably know Google changes the rules constantly.  It seems to me that whether or not reviews show up has mostly to do with whether or not Google has crawled the page where the review is and updated it's index to include the info.  It usually takes a few weeks from when the review is posted.

As far as ability to choose, sure they could, but they don't.  Google like to make algorithmic changes rather than individual moderation, which does make sense given the volume of data they are working with.

No stars, but they do improve your local seo rankings.  A few of my clients have had personal visits from Google local reps who have been able to get reviews re-posted.  With Yelp, the user usually has to write more than one review to avoid deletion.

These sites are getting reviews indexed by Google for canada local businessess

homestars.com
canpages.ca

There's really no way around it.  Google/Yelp have decided that unless a user is active over a period of time and reviewing several different businesses, the person must be a fake.  They can't tell the difference and commonly delete reviews.  Many of my local business clients have had legitimate reviews deleted. I've been recommending insiderpages.com since it is easier to get the reviews to stick and they still get indexed by Google.

Everything is worth what the buyer will pay for it.  Since organic search traffic ebbs and flows, a simple traffic times conversion equals monthly price may to be reflective of what someone will pay for it.   You might want to think about offering a lump sum payment where the customer takes control of the domain and a monthly fee where you retain control, but re brand the site for the client.

Good luck