Sometimes trade is profitable even if you don't have any advantage, so long as the cost of shipping is less than the profit margin at the market price.
If the cost of shipping is less than the profit margin, that means that someone is willing to buy something from you for more than it costs you to make it - which, in an ideal market, is pretty much equivalent to having a comparative advantage. (I think.)
You're correct. Simple example here. Transportation costs are added to other costs, and the same analysis follows from there.
Transaction costs are not really different from other costs, economists just like to highlight them because they're often forgotten.
I would like to learn more about economics but I don't know where to start. Can lesswrong suggest specific areas of economics that are particularly useful for understanding and optimising the world? Specific suggestions such as reading lists and resources would also be much appreciated.