What most Dealers are going to offer is about 2 to 3,K behind A-Plan for the 05 Mustang. I think your best bet would be to sell it out right. You will get more for it than trading. Sorry to say that but most Dealers are trying to steal The 2005 Mustangs right now and sell them for over original MSRP.
I also would ask the dealer if you could do an in and out or how much they would charge?
Once you find someone to buy your car. you would trade it in for the price that you were selling it for and getting the tax savings. Than he would be buying for the price you told him you would sell it for.
Let me know if you need any more help!