the ability to sell liquids to do such a thing

to buy and sell liquids not only in barrels but also simply liquidity

Balance-wise, I've decided to not introduce this feature at this point. I will be thinking about how I might integrate it though.

idea - barrels in liqiuds (angels mods) seall and buy

I ... what? You can already buy and sell barrels. Can you specify a little bit more?

yes mod angels - activate in functions - barrels

I still don't understand. If you mean that you can't buy some barrels, you might not be able to buy them because you have to reach the technology that unlocks the barreling recipe and also the liquids recipe.