you cant buy/sell from individuals... only through the public market can you buy goods from other countries...
if you really wanted to "trade" with a certain person/country... you could send each other Foreign Affairs packages to trade goods for cash... but that is going to cost you each 3 turns... and you need to be allied also... which costs a certain amount of money per turn to keep up the alliance... :)
It'd be an easier way to do buyouts. Buyouts with 16 countries and Standing Orders and everything else that's out there are already pretty easy. If they actually allowed you to sell to another country specifically, that'd be ridiculously easy.
On the bright side, all the people complaining about self-farming would have something new to complain about :P
Would like to take this opportunity to invite you to play in PANLV, a netting clan, every member of which is exceptional except for me. But I guarantee you will not only have heaps of fun with us but your every question will be answered by people who have been playing for years.
Do come and join us. We promise you a big welcoming party.
