Materia always sells well and it is pretty easy to obtain these days via various means. Also run the three elite hunt marks for a 5K gill each. The other levels also pay a 1K but can be a grind if you don't need Centurio Seals. If you haven't leveled your retainers do so for gathering and sell the proceeds. They usually return with HQ items and the newer ones go for decent amount on my server's MB. None of these are millions in one go but they add up pretty quickly.