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You can't buy BTS direct. You have to buy BTC or ETH etc and send to the DEX or buy BTS on Poloneix or Binance and send to the DEX.

The way I got started was to use @blocktrades service to convert the Steem Backed Dollars I earned here into Bitshares. I'm still doing that, but I use OpenLedger if the SBD gateway is up and running (sometimes it is down).