You can currently buy or sell CROATS from open source and decentralized application called Bisq or from STEX exchange.
If you want to read the details on how to use this application, you can visit the website of Criptonoticias and read the article they did.
Another channel to buy or sell CROATS is through the Telegram channel CROAT Trading
Bisq is a decentralized exchange that guarantees totally private exchanges. In this tutorial you will be able to take a look at its platform.
To learn more about how these platforms work, you can see the following tutorials: