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So they sell tokens to traders as trading fee, is that correct?

I googled Digitex X and one of the first results was a shady video referencing Bitconnect... I would stay away from that project if I were you, I smell scam.

So they sell tokens to traders as trading fee, is that correct.

No, the token belongs to the trader and that's the native instrument to be used for trading futures on their platform. Besides, that's not a new thing with exchanges; Binance for example. That video is an independent review. Why not look at official papers such as their white paper and press release.

But can you trade without buying the tokens?