EDIT: My Merchantmen can create steel, apparently. I had 15k steel and built 20 Merchantmen, which costs 15k steel. In their holds is 5k steel. Also, my damn Bulkfrieghters decided to come back with food, despite steel being the most profitable purchase they could have made - seems I didn't watch Kai/Vellos' Bulkfreighters enough before making my decision 
i would imagine part of the price in steel for merchant ships is that they sell some of that crap.
in any case, trade is bugged to hell and back. you literally create money out of thin air. proof? send a merchant ship to a planet with not much money and it'll sell its crap, zeroing all the money in that planet and then give money to that planet by buying stuff. .. ie.. it doesn't go -ve
i saw a bunch of merchants sold 90k q worth of steel (1k).. to a planet that didn't really have much money, certainly not 90k q... the planet had pretty much no steel and now has 1k+
it's linked, but i don't think other planets linked had that much money around either... i'm sure it happens in non-linked planet too..
