You can definitely sell high and buy back low. That is effectively what Crypto-ML is helping people do. However, when you sell out of BTC, you’re really just sitting on the side, protecting your money.
Short selling is a bit different. You’d sell on margin (e.g. sell Bitcoin you don’t have) and then buy it back at a lower price to settle. Since you’re selling something you don’t have, you are “borrowing” from the exchange (aka using margin).
Coinbase Pro used to support this and is talking about doing it again. But they do not currently. See the 4th FAQ here: https://support.pro.coinbase.com/customer/en/portal/articles/2936521-frequently-asked-questions-faq-
Here’s a post that dives into this a bit more: https://crypto-ml.com/short-sell-bitcoin/
Thanks! I really appreciate the questions.