I recently won a small war with my decades-long spiritualist rival to the north for two of their systems. I probably could have gotten more, but oh well that's what you get with Stellaris's war demands system. But I had two planets full of disgusting xeno scum and no real use for them, since I wasn't starved for minerals or energy at the time.

Immediately I start purging them off by default, but I soon had a better idea. I have enough Killbot armies on the planets to deal with any unrest from slavery, but again, I'm not REALLY in need of the economic stimulus they would provide since I'm pretty well off. But you know what everyone needs? Food.

Except for me. I had recently completed Synthetic Evolution and turned all my pops into perfect synthetic beings with no use for food. But you know who does need food? My old spiritualist friends to the north who's people I had recently taken.

So I switch my new toys from undesirables to slaves, and from chattel slavery to livestock. Then I strike up a very generous deal, giving my recently conquered neighbor 20 food per month for practically nothing. They accept, apparently not at all suspicious of why an entirely synthetic empire suddenly has a surplus of food production.

And that's how I sold an empire's own people back to them as food.