If a buy order is there one moment and I click sell and my sell doesn't fill due to the buy disappearing and this happens again and again, then Poloniex must be involved.
Again, no, because I have done that myself and I don't work for Poloniex. I have also demolished the price of coins while holding a substantial short position. Poloniex is simply very easy to manipulate. I am willing to prove it through live video.
You can observe another person making an order and place your order before theirs? I highly doubt it but if you think you can prove it then I'd like to see that.
The OP is not talking about people just withdrawing orders when they are getting eaten into by various people in bits, they are talking about signs of actual frontrunning - where they are removed due to knowledge of a single order and the ability to get in before it. That's not to say they are right because it could just be coincidence, but still...
