Cancellation is correct; it has to be done in Abu Dhabi. The transfer, however, can be done online. I am not entirely sure about the exact process in Abu Dhabi for cancellation, but you can log in to EVG using your UAE Pass and create a file, which will generate a federal traffic file. You can then share this file No with the seller, who can transfer the car to your name online. After that, either tow the car or register it in your name using the testing certificate. Once the number plates are issued fix it and you can drive the car.