With the exception of the Preface, all of Moll Flanders is told by, well, Moll Flanders of all people. That puts this book squarely in the first person point-of-view camp.
In fact, even he Preface itself is told from a first person point of view. It just happens to be someone else's: the book's actual author, posing as a character that has worked with Moll Flanders to get her story out there. Of course according to his own invented timeline, Moll Flanders would have been long dead before Defoe started writing, we can think of this author not as Defoe, but as yet another character he has invented to help tell Moll's story.
In each case, as in all first person narratives, all the information we get about the book's plot, characters, and ideas comes from, of course, that one person. So, whether it's Moll Flanders talking or the author of the Preface talking, we can only get our information from that one narrator. This means that whether or not we believe that person is telling the whole truth and nothing but the truth, that's all we have to work with.
So then what do you do if that person is a notorious con artist and criminal? It creates a little trust issue between us readers and Moll, don't you think? Of course the Preface doesn't help matters. In the second paragraph, the author-character tells us, "the author is here supposed to be writing her own history (Preface, 2). Uh oh. Supposed? So there's the possibility that it's not really Moll talking at all? What in the world are we supposed to do with that little tidbit?
This means we're forced to make our own way and figure out who and what to believe. Fortunately we're awesome readers, right? And we're also pretty tolerant, too, so we'll go along the lie that both of the authors are telling – that this is the true story of Moll's life as written by Moll – right from the first lines of the book. The more we get to know Moll, too, the less reason we have to believe that what she says is true.
But still, that seed of doubt has been planted, and the more we see of our narrator's rather shaky moral compass, the Moll seems like an unreliable narrator. She's a notorious criminal who successfully lies to and manipulates just about everyone she meets. What are the chances she's not lying to us?