I have done it, purchased a bike from a forum member who lived there; simple process however the most important bit is to get an export certificate, this was issued at the port and confirms the bike is being permanently exported from the IOM. AFAICR I only had to produce the V5 signed by the former keeper to get the cert issued.
You then complete the V55/5 and send it with the export cert and tax fee (road fund) to DVLA and you will get an age related number plate issued.
It was a simple process as I actually travelled to the IOM and rode the bike back, if not going in person then I guess the vendor would have to obtain the export cert on your behalf. If collecting it yourself you can insure it on the VIN until you get a new mainland number allocated.
I got a bargain as most people can't be bothered to travel or do the paperwork, I flew from Scotland to Manchester, took a train to Liverpool and went as a foot passenger on the ferry, picked up the bike and rode it home. Despite my travel and ferry costs the bike was still cheaper than any I could find on the mainland, so well worth doing for the right bike at the right price.