Marcus is right, best place to sell it is with Mark at PAS. Friendly guy who will steer you straight. I've had my 2004 up there for almost 6 months now. Car hasn't sold yet and my ad is about to expire and I've since decided to just keep it (not gonna lose money on it).
When I called and spoke to Mark before putting my ad info in I asked him what I should put it up for. At the time he said he could still get leftover 2004 S/C Coupes from dealer lots for anywhere from $28,000 to $32,000 (this was early in the year...April). I believe he said he actually picked one up for $28,000.
I originally put it up for $38,000 with a few calls of interest and three people came out to look at my car, then dropped it to $34,000 after 4 months with about the same interest. So, this should give you an idea. Mine is an '04 supercharged coupe with brake/wheel upgrades and 2,900 miles...pretty much same condition that your car is in.
Sorry to bring you the bad news, but the 2005's seem to be bringing down values of past Mustangs. Unless someone is dead set on a 2003 'vert in your body color...you're not going to have much luck getting what you want.
