Where do the rich men like Darcy and Bingley get their money from?

England was extremely class oriented. That is wealth was seldom self made rather inherited through generations of family lineage. Rich people died rich and poor people died poor, for the most part. Men like Darcy and Bingley come from "old money" or aristocratic families that trace their wealth through the generations. I'd be skeptical about the whole slave-trade idea. Trade probably meant items. Often trade included colonies of Britain (like India...). I think social class was Austin's point here rather than a moral statement as to how a specific family derived their wealth.