These are places you can buy/sell the various
Crypto coins.

A real simple one and a good one to learn on
is CoinBase.com. It lets you buy and sell Bitcoin,
Ethereum, and Litecoin. GDAX.com is owned by
Coinbase but has more tools and, like the stock
market, lets you buy the coins at Market AND
Limit dollar amts. If you do a Limit order and it
executes, there is no fee. That can add up when you
start trading more often.

There are lots of others...
Kraken.com, Gemini.com, Bittrex.com...