Learn English – Which is correct “The house is (being sold | selling)”

You can use one of the following:

He was told by police that they would try to find his motorbike.

He was told that the police would try to find his motorbike.

Police is treated as plural and usually "the police" but never "a police". If you were to refer to an individual police officer, then it would become an adjective to the noun officer, man or woman e.g.. a police officer; a policeman; a policewoman.

Regarding

The Joy of Not Being Married

First, keep in mind that lots of books have titles like "The Joy of _" -- it is something of an idiom (from The Joy of Sex to The Joy of X (a book about math)...). Like most book titles, these titles aren't complete sentences: they're just stating the subject of the book, which in this case is "What Makes [Subject] Great."

The form is "The Joy of [word or phrase that functions as a noun]". So you have books like "The Joy of Cooking", where "cooking" is (as you know) a gerund (an action considered as an object, so it acts as a noun grammatically), or "The Joy of Not Working", which is about the positive aspects of retirement.

In English, "marrying" refers to the actual act of getting married. So instead the author uses "being married", where "married" by itself is a participle, a verb form that acts as an adjective. Put it together and "being married" means "the state of having a spouse" and acts as a noun. With "not," it's a book about how it's awesome to be not-married. Maybe why it's great to be single, maybe why it's great to be recently divorced, etc.

Regarding

The Joy of Not Being Sold Anything

Again, this is a variation of the stock phrase. It would also help to point out that "being sold [a thing]" has the connotation of "being subject to someone's sales pitch" or "having someone try to sell you something." (See sell definition 4). So the ad is really just saying "Isn't it nice not to have to listen to a sales pitch here? Now please buy our product."

All this discussion aside, to actually answer the question you asked: you are correct that this is a sentence fragment. That's why "to be" is absent. (As opposed to what you might be used to seeing in full sentences, something like "He is being sold a donut.")