You must file Form 8300 to report cash payments of “over $10,000 received in a trade or business, if your business receives more than $10,000 in cash from one buyer as a result of a single transaction or two or more related transactions. … If you are required to file Form 8300 for a transaction, you must do so by the 15th day after the date the cash transaction occurs.”
The information in Form 8300 is considered valuable by the IRS and Financial Crimes Enforcement Network (FinCEN) “in their efforts to combat money laundering.” The IRS states that “this is an important effort, since money laundering is a tool that assists many individuals who participate in various criminal activities, ranging from tax evasion to terrorist financing to drug dealing, to hide the proceeds from their illegal activities.” 
Penalties for non-compliance: “If you willfully fail to file Form 8300, you can be fined up to $250,000 ($500,000 for corporations) or sentenced to up to 5 years in prison, or both.” 
Please note: This is not legal advice and dealers should always seek the assistance of qualified legal counsel.
From "19 Laws, Rules and Regulations That Can Cost You More Than Money" in the September 2010 issue of Auto Dealer Monthly.