What is the correct form to use for the sale of a classic car?

Assume this is a sale of a personal item and not a sale within a self-employment activity. 

Casual sale of personal items may be reported on Schedule D IRS form 1040 and Schedule 8949.

Personal items sold for less than their basis (most likely, the original purchase price) report $0 capital gain on line 7 of the Federal 1040 tax return.

Personal items sold for more than their basis report the gain on line 7 of the Federal 1040 tax return.  See this example.

                                              Selling                   Cost

                                              Price                      Basis

Item 1                                   $500                       $2,000                 Capital loss on personal item = $0 loss

Item 2                                   $400                       $300                    Capital gain on personal item = $100 gain

Item 3                                   $100                       $100                    No capital gain or loss

                                              $1,000                   $2,400

To report such a casual sale of personal items, in TurboTax Online follow these steps:

• Down the left side of the screen, click on Tax Home.
• Click on the down arrow to the right of Review.
• Click on the blue band labeled Review.
• Down the left side of the screen, click Federal.
• Down the left side of the screen, click Wages & Income.
• Scroll down to Investments and Savings and click on the dropdown menu.
• Click Start to the right of Stocks, Cryptocurrency, Mutual Funds, Bonds, Other.
• At the screen OK, let's start with one investment type, select Other, Land, Second homes, Personal items.

See also this TurboTax Help and this.