It was very difficult for a poor person to gain wealth. The only way to get property was to already have some, and occupations from which a person could get rich were closed to those of lower social standing. Basically, if you were born poor you staid poor. To get ahead, or even meet basic needs in some cases, you would have to rob.
I would argue that in addition to class inequities, Moll Flanders is a story of the inequities between men and women. Women did not have the right to own property, so the only way a woman could gain wealth or social standing was to marry a man who had it. A woman could also not marry if she was not pure, yet many men took advantage of young women who were either servants or otherwise poor, as in Moll's case. A man who had a relationship with a woman "ruined" her and ended her hopes of ever being raised in social class. Moll has this problem, of course. For example, when Moll has to get her husband drunk so he won't realize she is not a virgin.