What form do i use to report the sale of personal property, pneumatic trailer?

It depends - If you sold it at a loss, the IRS does not allow the recognition of capital loss on personal use items.

If you have a capital gain, then report it as the sale of a capital asset -

To enter the sale of a capital asset in TurboTax, log into your tax return (for TurboTax Online sign-in, click Here and click on "Take me to my return") type "investment sales" in the search bar then select "jump to investment sales". TurboTax will guide you in entering this information.

1. Choose “Stocks, Mutual Funds, Bonds, Other” and select “start’ (or “update” is you have already worked on this section)
2. The first screen will ask if you sold any investments during the current tax year (This includes any sale of personal property held as an investment property so answer “yes” to this question)
3. Since you did not receive a 1099-B, answer “no” to the 1099-B question
4. Choose the type of investment you sold - Personal Items
5. Some basic information:
1. Description –  type of vehicle
2. Net Proceeds – Total amount received for the property less eligible selling expenses (if any)
3. Date of Sale – Date you sold the vehicle
6. Tell us how you acquired the property
7. Any Business or Rental Use - If the vehicle was used for personal use only, you will not be able to deduct the capital loss since no capital loss is allowed for a personal use capital asset.
8. Enter Cost Information:
1. Date Acquired –   Date you purchased the vehicle
2. Original Cost – Cost of vehicle plus any capital improvements