I would highly suggest not doing this on Coinbase. Primarily because the Fee's are going to eat away at your $$ fairly quickly.
With only $100 to invest, I would highly suggest using a platform like Whaleclub.Co. When you trade here you trade on margin so your not necessarily going to be buying and selling BTC to USD when you trade but long or short your current BTC balance based on whether or not you think it will go up and down. Your profits will remain in Bitcoin until you send your coin back to a place like Coinbase to sell for USD, so if your currently bullish on BTC as a whole your profits will continue to make profits assuming BTC continues to rise.
Long = Will go up
Short = Will go down
They offer a lot of other currencies to trade as well as stocks, commodities, and bonds.
Whaleclub's also pretty cool in that they give you a 5BTC demo balance that you can use to learn their platform and how to trade. I always tend to do really well when I trade demo LOL
I've used WC for many years now and its one of better options for margin trading IMO. No ID verification required and the interface is clean and easy.