For buying and selling, while it depends where you live, my preference is Gemini Active Trader. While it may take a little bit of learning to use it if you've never used any kind of exchange before, it's worth it to learn how, for a few reasons.
One, it'll get you a 90% or so reduction in fees from "beginner" platforms, which means more profit for you.
Two, it'll really only take an hour at most to learn.
Three, it's just a damned good life skill. You'll be grateful the rest of your life that you learned the basics of a financial exchange.
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