It was discussed in a fairly recent thread that after founding Sushido that they founder was unable to trade for Sushido resources (simpy being "redded" out).

A few theories were offered for the AI prohibition, but I've now found the answer.

Other than simple diplomatic prohibitions (An AI simply hating you so greatly that they wouldn't trade a bubble gum wrapper to you for a million gold) the key to unlocking those "redded" resources is to simply expand Sushido into a second city. Once the AI sees that it exists in two of your cities the "red" disappears.

Presumably the AI isn't recognising that the HQ uses resources as well as branches, hence redding out due to it thinking duplicates are useless to you.

So you could probably trade with a corp in only one city as long as it isn't the HQ (i.e. a foreign corporation). I'd never let an AI keep Sushi for long though...

Right. On both accounts!