There was a gil making guide on GameFaqs. I don't recall the specifics but it involves buying an inventory full of a certain kind of armor, fighting one of the bosses in the Monster Arena repeatedly to get the junk to put a certain effect on all the armors then selling them back to a merchant. Repeat ad nauseum. Also while stat maxing a number of the bosses in the Monster Arena drop weapons consistantly that sell for quite a bit. The armor bit is faster but you can fight the bosses and stat max while earning gil.