Best place to start is at car max, they will give you a quote good for two weeks. If it is good enough for you sell it to them, if not FSBO.
Carmax. It will prob be a little low, but it's a great reference number. If its what you're expecting, and worth the savings in trouble, take it.
Dealers will def try to lowball trades, but it if you're not great at dealing with dealers, it's another negotiating point. They do make it easy, and you have the ability to save taxes on the new vehicle.
FSBO is best, but the most hassle. You'll get the most, and if you do the paperwork at the dealer you're getting your new car, you can still save the taxes. I sold my truck on CL, but it's a bit of a pain and you have to be careful.