So I'm thinking of selling my 2009 Nissan sentra....?

Sorry to inform you, Nikki, there is not a buyer on the planet who will give you $6k for your Nissan. A 2009 Nissan Sentra SL in very good condition with 80k miles has the approximate current market value of $4,400 from a private seller. Even worse, its trade in value is only around $3,300, tops.

https://www.edmunds.com/nissan/sentra/2009/st-101109360/appraisal-value/tmv/

You obviously paid more than the car is worth when you bought it. The BEST you're going to get from a 9-year-old used Nissan Sentra is $4500-$5000. Besides... You don't NEED a "faster" car. Your Nissan is fast enough to exceed any legally posted speed limit you are likely to encounter. Don't abuse it, maintain it properly, and it will remain that way for a good long time to come.

Keep what you have. Lucky to get $3000 on a trade, $3500-4000 via craigslist. BUT, can take months to find a buyer.

A faster car will kill you on insurance. Sentra is PLENTY fast enough. I sold one with 200,000 miles and the guy who bought it had it going 85 on the test drive. 85 is PLENTY fast.

When you buy a car, the dealer makes $1800, the salesman makes $300-400. You have fees & taxes and you don't get any of that back.
Plus your car has depreciated $1000-1200 over the last 8 months. Add it all up. And your $6000 car might be worth $3000.

Where you list it for sale isn't very important. Whom you find to buy the car is... To get top dollar for your vehicle, make sure it is cleaned inside and out, look after any small details such as paint chips/scratches, burned out bulbs, any warning lamps on the dash, etc.

When I search for new vehicles, I look through various sources. When I sell my cars, I put up multiple postings on different sites.