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Poloniex does not have 0% transaction fees. Each buy and sell does cost between 0.15% to 0.25%, so I'm not sure how much is bogus volume.

Now painting the tape with bogus buy and sell walls would be another conversation. Unless the order executes, no fee is paid. That is common practice on every crypto exchange I've been on and I doubt you'd find one without it.

I know they don't. It doesn't mean that the same sort of thing doesn't happen. If you can make up the losses on fees by manipulating the price then it might be worth it.