I am renting out my car using Turo. As part of providing the service, listing and insurance Turo takes 25%. Is this fee a deductible expense?

[Critter]

Yes and no ... depends on  how they report it.....important thing is to match what they report to the IRS because if you don't that is an IRS audit waiting to happen ... so .....

If they report the gross income to the IRS (usually on a 1099-misc) then you will report the gross income and you may deduct the expenses on your Sch C along with any other costs. 

If they report only the net income to you  ( the fee - the 25% commission/cost) then you only report the net income  ... you may not deduct the costs again since they did it for you already.

[Critter]

If you have net self employment income of $400 or more you have to file a schedule C in your personal 1040 return for self employment business income. You may get a 1099-Misc for some of your income but you need to report all your income.  So you need to keep your own good records. Here is some reading material……

IRS information on Self Employment….
http://www.irs.gov/Businesses/Small-Businesses-&-Self-Employed/Self-Employed-Individuals-Tax-Center 

Publication 334, Tax Guide for Small Business
http://www.irs.gov/pub/irs-pdf/p334.pdf 

Publication 535 Business Expenses
http://www.irs.gov/pub/irs-pdf/p535.pdf 

There is also QuickBooks Self Employment bundle you can check out which includes one Turbo Tax Home & Business return and will help you keep up in your bookkeeping all year along with calculating the estimated payments needed ....
http://quickbooks.intuit.com/self-employed