When it comes to the headers, a dealer won't care. If you're selling privately, it might be worth a few more bucks to someone. (The idea of putting expensive mods on your car makes it worth less is ridiculous, by the way. I've always sold my modded cars at a premium. It's all about the right buyer.)
Anyway, if you're trading the car in, take it to them and get a value from them. Then, take the car home and swap the stock manifolds back to the car. (Tell them you're not leaving them on the car up front. Always be up front and honest in your dealings.)
Otherwise, good luck with the transaction.
