The best bet is to find the boat. lots of looking, lots of boats, I have bought 2 boats over the last 40 odd years, one from a Marina' years ago, good service, good advice, one from a boat broker, good service, but bear in mind they charge about 4% commission,. Sold 2 boats, one through an odd introductory source, straight fee £500. but a good deal for both vendor and purchaser. The second through a private advert...........sold within a week.
People frown upon private deals, but I approve of vendor surveys, I don't think deposits are a good idea and try an provide as much info as possible.....I don't believe in internet sales of boats, always look for good 'paper trails' of receipts for work, ownership, licences, safety certificate inspections and walk away if you are not happy with any aspect of the deal.
I also walk away if any part of the dealings are in a language other than English.