This is very easy if you have Cash App. Their service allows small purchases (<$1000) on a regular basis. When you buy, you don't even need to wait for confirmations for the coin to hit your Cash App wallet, and you can immediately transfer it to any other external wallet. I'm not 100% sure, but I believe you can configure ongoing, scheduled purchases to build your Bitcoin holdings. And selling is just as easy. Just transfer transfer bitcoin to your Cash App wallet, and you can immediately sell at the current price, minus some small service fee. I think Cash App is the easiest way to buy and sell Bitcoin.