Can you get money from trade between 2 provinces you own?
No. Internal trade is the value of the trade item times the trade building multiplier, external trade is that number for all neutral and allied provinces that are connected by an unbroken shipping link. You cannot trade an item to a province that has the item as one of its trading products.

mfberg

ps. There are extensive discussions in the econmics in MTW (Economics 101) thread, including which provinces to own and what buildings to build for your economy.