Craigslist now charges you to post a vehicle for sale. You should have no issue selling on FB marketplace.
Like others have said, cash or cashiers check only. Cash in hand if you want to take it for a test drive.
Clean the car to the best of your abilities, full detail, and take as many pics as you can (tire tread/rust/engine bay/seats/dash & odometer with car running (dash lights)/seats/exterior). Sometimes you will have people ask for specific photos other than the pictures listed and it's always nice to have clean photos already on your phone. I don't believe a bill of sale is actually required in Iowa, but like others have said it is nice to have, and including "as is" language doesn't hurt.
I tend to be as descriptive as possible on a post because the public is dumber than you think and will ask you questions you already answered on the post, so I will redirect them to the post to weed out any people who like to waste time on marketplace. If you only post years/miles/"runs good" you will be answering the same 5 questions multiple times a day for the next few weeks.
Look for comps online, KBB might give you a good place to start, price-wise, but since there is more of a demand for used cars right now, KBB could be low. Probably start at $1K over the lowest price you would take.
And for the love of god, don't worry about covering your license plate in the photos.