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.")
because I think it can't sink by itself
When you think that, it's always a good idea to check a dictionary.
Merriam-Webster tells us this about sink:
intransitive verb
1
a : to go to the bottom : submerge
[...]
transitive verb
1
a : to cause to sink
As you can see, the verb can be intransitive, meaning that ships can sink by themselves. There is nothing wrong with "the ship sank".
At the same time, it can be used transitively, in the way you considered. Someone can sink a ship, and a ship can be sunk (by someone).
Best Answer
You are right!
Since the sentence uses have helped, the suppose to phrase can be rewritten the way you said.
In such context, the phrase supposed to is used for the action that actually does not happen. EnglishGrammarSecrets mentions it:
Mind it that if you use be supposed to without mentioning any action in past, the meaning changes. It then represents something which is expected. Good tutorial here.