In researching this exchange, we found that the company registered as the operator of this exchange in the UK is Exchange Cloud Limited, and the person in control of that company is Oluwasegun Oni. A review of his LinkedIn shows an extensive work history but no mention of this exchange. If the exchange was as successful as they claim, we would expect him to proudly display it in his work history! That said, our research did not turn up any other suspicious behavior on his part, so far.
ExtStock is also accused of reporting inflated volume. Their bitcoin price is higher than on other exchanges, which is inspiring users to transfer their funds to the exchange in hopes of selling at a profit, only to find their BTC locked into the exchange due to the aforementioned accusations of "suspicious activity."
While we cannot find conclusive proof that ExtStock is planning an exit scheme or suffering from a lack of reserves, we do find their behavior concerning enough to mark it as a SCAM and warn users not to transfer their funds to this exchange.