I've sold a bunch of cars on CL and I have USAA. Here is what you do.
Post your car with a good description and lots of pictures. You should start getting responses. There are lots of flaky people out there so be prepared to return txt messages or people asking you questions that you had already answered in your posting but then never hear back. You may get called by businesses who want to sell your car for you for a cut of the profits...these people suck. This was the only scammy type of response I got.
After an interested person wants to see it, you schedule it with them and meet somewhere open and public. I choose this park near my house that always has people there. The prospective buyer usually comes, looks over the car, then wants to go on a test drive. Have a short route in mind. After you get back from the test drive they will usually circle the car a few times then either talk you down on the price or tell you they will think about it and get back to you. Sometimes you will never hear from them again while other times they will txt or call you for a lower price later in the day. If they want to talk about price immediately then that is usually good since they should've done their research. Now here is the crap part...be ready for an offer anywhere from $500 under to $2000 under your posted price. People are punks. I will negotiate with them as everyone wants to win a little bit but usually in the end the price will end up anywhere from $200 to $600 lower in my experience.
After a price is negotiated I usually arrange that we meet at the buyer's bank and we walk in together and they get a cashier's check for me. A few times the person pre-processed the cashier's check while I wasn't there and I went inside and verified with the teller that the check was legit. After you have the check in your hand and it's verified, you sign over the title and fill out any other 'bill of sale' paperwork that your state requires...then deal is over.
With USAA, I found 3 UPS Stores nearby that will do USAA check deposits so find one that has the USAA Check scanning machine and use this method to deposit the cashier's check in your account.
I once had a guy pay me $13k cash. What I did was go into any bank, pay a small fee like a couple of bucks, and get a cashier's check written out to me. If you have a family member with a BofA account i think they can get a cashier's check for free.
Depending on where you live though, I would recommend bringing a friend to the meeting place where you will show the car. I went solo each time but there were some shady folks on a few occasions.