First of all, IMHO, yours is worth far more than $8500. I would say that a well-maintained '96 coupe with 70K miles is more in the $10,500 to $13,500 range. Corvette Magazine lists the prices for a 1994 Coupe in their C4 price guide, (July 2005), as $7,100 for "Crummy", $9,500 for "Decent", $11,600 for "Great", and $14,400 for "Fabulous". With 70K miles and if you have good records, minimal/normal wear on the interior, and tires w/50% plus life I would certainly put yours above the Decent category and the better the condition of all wear items could move you even past the Great price level.
Selling your car yourself is really not that hard with a simple ad in the auto trader and/or local paper usually doing the trick fairly quickly if you list at a fair price with good description. Never ever, ever never, trade your car in at the dealer, (dont kill me car dealers out there!), as you will always leave money on the table. Especially with a car like the Corvette that will reasonably sell fairly easily in most situations.
Secondly, the '03 sounds a bit high given the bargain level prices, (under $40K), that were available for coupes at the end of the '04 model year in anticipation of the C6 arrival. Late model C5 prices have eroded well this year. Corvette Magazine's C5 price guide, (September 2005), lists the top price for a "Fabulous" '03 Coupe at $33,500 and "Great" one at $30K so I would more reasonably shoot for the low 30's, ($30K-$33K), for an '03 coupe with 11K miles like the one you describe. 11K miles certainly isn't excessive mileage but not necessarily extremely low for a +/- 2 year old vette. There were 2086 Medium Spiral Gray Coupes produced in 2003 which was actually the 2nd most popular color for coupes that year behind Anniversary Red so I would not say that the color warrants any extra cash.
I believe that you could reduce the overall costs of this deal by $6K to $11K if you sell your car yourself, (net $3K-$5K more), and negotiate a little harder for the '03, (save $3K-$6K). Just keep the new purchase separate from the sale of your '94.
And if you decide to let them have it for $8500 send me some quick pics and directions to your house, I'll be there to pick it up the next day and give you $8600! Good luck with your newest Corvette!