You can never have too much, when it comes to resources. You'd be surprised how quickly your walls will eat that stone, and how quickly research will eat up that gold. As long as you always have something queued you should be fine. Also it is somewhat hard to tell you what to sell for what without knowing your resource field makeup, marketplace prices, levels of stuff you've built, etc. My best advice is to look at those areas, as well as plan ahead by checking out what upcoming projects will cost you in the future. Good Luck to you!!!