Unlike the great epics or even Shakespeare's plays, this novel presents us with extended insight into the thoughts and motivations of the main character. One of the best things about a novel is the ability of the writer to create character development, and here, Defoe presents us with several realistic situations of a common-man type character who, at least in some part, represents "real life." There are no monsters to battle with superhuman strength. There are no gods intervening. There are no supernatural events to contend with. When trying to argue "verisimilitude" recall the most common of details about her life, the world she inhabits, her interactions with other characters, and you will have a lengthy list of the kind of details you are looking for.
Moll Flanders is one of the earliest examples of a novel. It is a unique book, and is told very realistically with a lot of detail. It is a story of love and loss, but it is also a story of everyday life. Written in prose instead of verse, and focusing on the common life of everyday people, this book helped establish the novel as a viable form of literature. Up until this point most literature was fanciful and told of the hero and his adventures, or about nobles and kings. Defoe changed the landscape of fiction forever with this book.
I think focussing on the struggles and joys of Moll Flanders, who can be viewed as one of the first heroines who uses her sexuality to her own advantage, gives this great novel definite verisimilitude. Note how Moll Flanders is not a traditional heroine in any sense of the word. This novel gives a distinctly realist flavour by its focus on the seamy underside of life at the time, in which Moll dwells.