Appending a question mark to a declarative sentence results in a valid sentence?
(This is just to elaborate and provide some examples. I stand by the assertion above. Here's why.)
1. (Linguistics) a sequence of words capable of standing alone to make an assertion, ask a question, or give a command, usually consisting of a subject and a predicate containing a finite verb
[Collins English Dictionary, unabridged]
A valid sentence, then, need only fulfill one of the requirements listed above. Although we tend to think of sentences the way we were taught to in grammar school (i.e., involving at least a subject and a predicate, blah blah) a sentence doesn't have to be defined so narrowly. The following are all valid sentences:
Joe: Going to the mall?
Tom: No, not really. Just messing with you.
Joe: Oh. You bastard.
Tom: I'm being a bastard?
Joe: Well, if the shoe fits ...
Tom: The shoe fits?
Joe: It doesn't?
Tom: Well, maybe.
Tom: All right.
Joe: So. The shoe fits?
Notice that peppered in the dialogue are declarative sentences expressed as questions. Sometimes they are used to cast doubt on the assertion, but in Joe's final line it is used as a request for affirmation.