he doesn't want your account number. He wants your cash. He cuts you a bogus/stolen/forged check for $8700. You send him a good check for $4200 ($8700 less the $4500 sale price). He winds up with $4200 and you wind up holding the bag when the check bounces or is finally identified as stolen/forged.
The boat is immaterial to the scam.
Also the use of Western Union identifies this as a variant of the Nigerian/419 Scam -- Section 419 of the Nigerian Penal Code being the section pertaining to mail fraud, and Nigeria seeming to be where the scame originated and most of this stuff comes from. People have been murdered after getting hooked into the Nigerian scam.
The reason for using Western Union is that the recipient of the cash doesn't have to pony up any pesky personal details such as bank account numbers, etc. All the recipient needs to get the cash is the control number and the answer to the secret question provided by the sender.
You should contact your local postal inspector and the police (if the money moves through the mails, its mail fraud, if it moves by wire its wire fraud. Either way its a felony.)
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