Can I report the sale of a personal item with a loss on the online form?

The loss on the sale of personal property is not reported on a tax return.

Yes.  You will enter the same way you would enter an investment sale reported on a 1099-B.  Do report in TurboTax Online:  

• Type 1099-B in to Search and hit enter
• Click on "Jump to 1099b" 
• Answer  Yes  to "Did you have investment income in 2023?"  
• Click through the program until you get to "Let's import your tax info?", and click on "Enter a different way". 
• Click to highlight the box with "Other (land, second homes, personal items)", then click on Continue.  
• Enter something for "Name" and click on Continue.  
• Click the down arrow to open the drop down menu for "What type of investment did you sell?"
• Select "Personal Items" 
• Answer the questions and enter the details of the sale

FYI, although a gain on the sale of personal items is taxable, you cannot deduct a loss on the sale of personal items.