Since you are 17 (thanks for telling, I am geezing around in my 50s
) it actually makes less sense for you to sell the car unless you already have the other one bought.
Since you are obviously single and living with family that kinda obviates the utility of a 2nd car.
G35s are often fantastic used car buys (though as with any car, lemons do exist on the used car market so exercise some care ,especially since you likely don't have a lot of experience buying used cars..dont just buy the first one, also I am sure there is a used car buying guide here somewhere on Nico)
So ultimately my advice to you is this.:
You are 17. you dont need a hot car to retain decent status amongst your peers.
If your Sentra has been good to you.. it may be more important to your lifestyle to have the extra $$ in your pocket that buying a nicer car will take out of your pocket.
When I was younger I kinda lived in "Ricer wonderland" where every single dollar I made went into buying the next nicer car.. as a result I was always broke.
And chicks do not dig that at all.
If your car is reliable and road trip season startiing soon just having plenty of money for gas is a treasure in itself.
Take a couple of buds on a 1000 mile road trip to Appalachia or drive up to the Michigan upper peninsula.. camping..
etc etc
Then you are not driving a car thats expensive to you ...and you will not stress putting a lot of miles on it.
Then later in the year when you are working again and
have enough money in your pocket that the low ball price you will get for the sentra wont bug you.. then and only then, buy that used sweet, G35
When the lowball price for your current car bugs you.. either financially or emotionally, then thats another sign you should not buy that next car just yet.
PS: When you go on that road trip, take a lot of pics of the car and he trip and post 'em in the sentra forum!