I will answer the question in this topic title: To buy DOT, you need an account at the exchanges: Binance, Coinbase, Huobi, Bithumb. Then you can use USDT or other crypto to buy DOT there very easily. DOT price has gone below $15, this is a good time to buy DOT if you trust the PolkaDot ecosystem.
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HOW TO BUY POLKADOT (DOT)
Polkadot (DOT) is an open-source sharding multichain operating on the Ethereum network. Polkadot uses oracle technology to connect blockchains together, allowing information to be shared between multiple projects and networks. The goal of the Polkadot network is to create a truly decentralized web by allowing blockchains to “communicate” with each other. Polkadot is currently the 9th largest cryptocurrency in the world when measured by market capitalization.