When you operate outside of the law, you bear the risks of that decision.
When you operate within the law, you have a number of avenues, such as the courts and police to mediate disputes or other problems.
money.stackexchange.com/users/1288/duffbeer703
If you get counterfeit money, then you're dealing with the criminal who is going to be punished by the law for doing that.
The portion of the total sum that was paid with the counterfeit currency is considered unpaid and you can claim the money from the criminal and sue him, while he's in jail.
He'll work hard on those license plates to pay you off.
However, making false statements and assisting in a tax evasion scheme compromises your ability to go to the law enforcement in case of any wrongdoing, and then you should worry about the counterfeit money, because the law won't be on your side to help you.
And you don't even get anything out of it...
Why on earth are you willing to take this risk?
Just so you know, it may also be money laundering, which may get you in trouble even more with the law.
money.stackexchange.com/users/2998/littleadv
I'd not do business under these terms.
A bill of sale needs a signature, right?
Your signature is your word, and your word is your bond.
I wouldn't participate in such a fraud, nor would I accept this sum of cash, who knows its origins?
money.stackexchange.com/users/187/JTP - Apologise to Monica
I'd worry more about falsifying documents of sale.
No good reason at all to do that.
Detecting counterfeit bills is easy if they're all new bills.
Hold them up to the light and look for the watermark and the numbered tape in the bill.
Refuse any bad ones.
money.stackexchange.com/users/414/mbhunter
I'd be a bit concerned about someone who wanted to transact that large of a transaction in cash.
Also consider what you are going to do with the funds, if you deposit it, you will need to tell the bank where it comes from.
Why does the bank want to know, because most legal businesses don't transact business with large sums of currency..
What does that tell you about the likelihood the person you are about to do business with is a criminal or involved in criminal affairs?
The lower bill of sale price might be more than just to dodge taxes, it could be part of money laundering..
If they can turn right around and 'sell' the boat for $10K, or trade it in on a bigger boat for the same amount, and have a bill than says $4K, then they have just come up with a legal explanation for how they made 6 grand.
and you could potentially be considered an accomplice if someone is checking up on their finances.
Really, is it worth the risk.
money.stackexchange.com/users/3924/Chuck van der Linden
Paying tax is a Good Thing.
However, warren has made good point
and I would like you to consider this other thing: Go into your payees bank with the payee, get the money withdrawn from the teller and take it with you.
Unless I am missing something, or the teller handed your payee fake notes, you are "safe"
money.stackexchange.com/users/4189/f1StudentInUS
The only issue I can see is that the stranger is looking to undervalue their purchase to save money on taxes/registration (if applicable in your state).
Buying items with cash such as cars, boats, etc in the used market isn't all that uncommon * - I've done it several times (though not at the 10k mark, more along about half of that).
As to the counterfeit issue, there are a couple avenues you can pursue to verify the money is real:
marker pens (used by stores a lot of time even on 20s) UV scanning not signing the title over until the money has been deposited at your bank alternatively, putting the title in escrow for the time it takes to make the deposit
/ have the bank verify the funds
this could be as simple as doing the purchase in the bank parking lot, walk inside, make deposit, sign title to buyer
*
it's the preferred means of payment advocated by some prominent personal financial folks, including Dave Ramsey
money.stackexchange.com/users/969/warren
I would not do a bill of sale for less, but a legal and safe way to reduce the taxes is to write separate bills for the boat, motor and trailer.
The taxes are paid at different rates and will represent to full sale price.
money.stackexchange.com/users/52207/Butter13
I'm selling a boat through a broker (a friend of a friend) taking a consignment.  The buyers offered to pay in cash, but want the bill of sale to say less than the actual price for tax reasons.  Should I worry about counterfeiting if it is such a large amount? Or should I worry more?
If accepting more than $10K in cash for a used boat, should I worry about counterfeiting?
money.stackexchange.com
<cash><selling><used-goods>
money.stackexchange.com/questions/9250
money.stackexchange.com/users/4046/Boater
You should be aware you would be falsifying the documents which could get you into trouble. In addition to that, it could be money laundering and you would need to be able to prove to your bank where all this cash has come from. The lower bill of sale may be due to money laundering, which will get you into trouble for falsifying documents of sale. When you deposit the money, you will also need to tell the bank where it has come from. There is also the risk that the money may be counterfeit. You can detect counterfeit bills by holding them up to the light and looking for the watermark and number or mark them with pens.
If you receive counterfeit money, you are probably dealing with a criminal and I would also worry about falsifying documents of sale. The lower bill of sale may be money laundering, which will get you into trouble. You can detect counterfeit bills by holding them up to the light and looking for the watermark and number tape. You can also use marker pens. Also consider what you are going to do with the funds, if you deposit it, you will need to tell the bank where it comes from.  