Before bidding on on boat on ebay confirm with the vendor that the bid can be subject to survey. I have surveyed too many boats that people have bought from various sources without a survey and found out they have wasted thousands of pounds.
Boats to be used on the inland waterways will need a Boat Safety Scheme certificate, again see sight before you hand your money over. If the boat has been used on the sea where the scheme doesn't apply it might cost a large amount of money to get the vessel to the scheme standards.
Surveys can cost from around £150 for a small boat hull survey and usually find some faults which may result in being able to negotiate a price reduction.