It's the difference between doing it yourself - ie taking cash / bankers draft etc and employing a Broker to do it for you.
As part of ABYA - Association of British Yacht Agents membership you are required to have a dedicated and properly managed clients account.
Vendor agrees terms with Broker - commission rate, storage costs, specification, marketing plan etc.
Buyer finds advert.
View boat. Like it.
Make offer Subject to survey.
Haggle a bit
Agree offer subject to survey
Pay deposit, normally 10%, into brokers client account.
Have survey and/or sea trial. Lift out costs and surveyor costs for purchasers account.
Maybe Haggle a bit more if there are faults that have come to light that weren't prevously known. Come to an agreement on final sale price.
If not done already the Vendor should then pass to the Broker the title paperwork. The Broker will then check this out to see that it is legitimate and proves that the vendor has clear title.
Transfer balance to Brokers client account.
When monies cleared into Brokers client account boat is released to Purchaser.
Sale price less brokers commission then released to Vendor.
In the above at no time do either party (Vendor and Purchaser) hold all the parts of the deal - ie no-one has both the boat and the money and the ownership paperwork.
If the boat is found to be in poor condition at survey or a major fault found that means that you no longer wish to proceed with the sale, even at a reduced price, the Broker will return your deposit. This may be less certain costs such as skippers cost, fuel cost etc (Which the broker would probably have swallowed if the sale went ahead).
For an Escrow system to work you'd need to have a contract detailing all of the above in place. Solicitor + escrow account is probably more expensive than having a Broker do it for you - and the Broker advertises the boat for free!