If part exchanging, just let the dealer quote their price (and haggle if required). If selling privately, as already said just be honest about the faults.
Personally, I would part ex it at the dealer - getting them to check it and keeping your mouth shut will probably get a similar price to selling it privately and being open and honest, with less hassle.
I will be selling my old car in order to get a bigger family car. I was wondering if it was best to get little niggles (like slightly leaky window seal) sorted out before selling it or just to sell it straight off? I am looking at going through a car dealer.
She's quite a well travelled little car with around 105000 miles on the clock, Peuguot 206 diesel.