As you know, you are required to report to the IRS, using Form 8300, when your dealership receives over $10,000 in cash from a consumer. Note that what is considered “cash” for reporting purposes and how to make this determination can be complicated. See the IRS Instructions for Form 8300 for details.
It is also important to remember that when a Form 8300 is filed, you must also provide a written or electronic statement to each person named on the form on or before January 31 of the year following the calendar year in which the cash is received. The statement must show the name, telephone number, and address of the contact for the business, the aggregate amount of reportable cash received, and that the information was furnished to the IRS. Be sure to keep a copy of the statement for your records.
A statement is considered to be furnished if it is mailed to the identified person at the identified person's last known address. These requirements can be found at: 26 C.F.R. § 1.6050I-1(f).
NADA has more detailed information about Form 8300 requirements here.