[patrickk]

> What's to stop them from selling it to each other?

Nothing. This is what we call luxury goods.

> Could we see a completely stratified economy, with rich people selling each other the fruits of their automated factories' labour?

Already exists to an extent: http://en.wikipedia.org/wiki/Hourglass_economy. (strong high and low ends to the market and hollowed out middle.)

There's challenges: high-end luxury goods that are totally mass produced don't tend to go together, as it feels too "mass market" or low quality (Ferraris have hand sewn leather in the seats, so they are high quality/"feel authentic"). Apple combines mass production with high end pricing/high quality successfully, but they seem to be an outlier to an extent.

The number of ultra high end consumers is by definition quite small, so there's considerations if a company or brand wants to grow beyond the high end of the market.