So basically, you're looking for a platform where you can sell cryptocurrency and buy back later.
You should know there are differences of Buy & Sell; Long & Short; Bid & Ask.
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Sell now and buy back later, it is Buy and Sell. Short relates to Long and leverages so it relates to Short squeeze that is terrible. Crypto market does not operate only because of supply and demand but is affected seriously by Long squeeze and short squeeze.
So if OP want to short a cryptocurrency, please zoom in and out to have narrow and wide view, and think of short squeeze risk.