Welcome to the Forum.
Honesty is the best policy, so declaring the fault will give you peace of mind.
Take time to clean the car, and get all your paperwork ready is the first step.
Gumtree will let you advertise for free. Don't waste money on the premium listing they offer. To keep your listing near the top, cancel it once a week and relist for free! Nice set of clear photos and a well worded add and you're off. Check Autotrader/ebay etc to see what others are selling for to give you an idea of price point.
A4 For Sale sign in the window and park it somewhere fairly busy works too.
Forum has a for sale section.
Good luck with your sale.