Okay, so I was asked about trade, specifically trade of grain for livestock.
Without markets and ports or caravanserai, traders and merchants aren’t terribly interested in trading this. The infrastructure simply isn’t there. The same is true of ‘trade goods’. They would be forced to take very dangerous routes through wilderness and haul crates up and down shores, row through dangerous waters, etc.
As you develop a ‘trade’ attribute, from having inherent ‘trade leader attribute’ or building ports and markets and that sort of stuff, your infrastructure is developing alongside it. Building roads, mapping coastal trade routes, etc. That’s partially what your ‘trade’ attribute represents.
Then it becomes advantageous for your traders and merchants to trade goods. If you are producing trade goods (in your city or across your empire), you will begin to make money in your markets, ports and caravanserai, etc.
Now, if you want to trade things like livestock or grain, you can do so but your traders don’t see enough of a profit to do so on a large scale and certainly there isn’t enough profit in it to make large-scale taxation profits. Now, that many not be terribly realistic, but it does simplify things by not having to give trade goods for things like cattle farms or grain farms. At trade rank 6 (or growth and development rank 6) you may want to design a building that produces trade goods form livestock- maybe a livestock auction or some such. That’s your call and I encourage you to do so. Side note: rank 6 and higher buildings and troops are up to you to design genereally.
Finally, if you want to just trade another player your ‘food’ for ‘gold’, you can do that to. You won’t see profits on the trade but you will receive the ‘food’ or ‘gold’ from the other player. Depending on your ranks in ‘trade and exploration’ you may lose some of the traded goods as you go. Note that you cannot trade hammers (ever- this is contrary to what I wrote earlier).