I prefer to sell myself. There are THOUSANDS of dollars to be made by doing it yourself. Pay for ads, get free ads, but advertise that crap out of it.
You'll get a ton of calls from people who want to help you advertise it some more -- ignore them. If someone calls and asks, "do you still have the vehicle for sale... they have no idea what your selling or care. They're just ad scammers.
A Civic Si will sell quickly. Don't let the dealer have YOUR money.
Since you owe, you'll have to work with the buyer to get the title after they give you the check. I think Capital One has a deal where they offer some escrow services and the UPS store for notarizing the deal (but it's been a few years since I went that route so check the details.)
Dealers make it easy for you to trade in a car. For good reason. Profit.