I'm going to move this over to the Boat topics, since it's more of a general boat buying question than a Volvo repair question. Boats and engines of any kind (IO or OB) develop problems with sitting around and not being used. Especially if it was used for one year, then left sitting for the rest of the time. Fuel problems, freeze damage,.
Get the boat surveyed, and a mechanical inspection, and do not buy it without a test run.