it's not so much that the value on Vettes in general are going down, it just that certain stles or years are currently in a downward movement but they will eventually find a new stable price point although at a lower price than current values.
C4's in particular and C5 resale valuses are down right now mostly I think because of the new C6. C4's have been going down for a while and are right now probably the overall best value in the Corvette market if you are looking to buy a car (but not to sell one). C5's have been going down because of the C6's and dealers dumping their remaining C5's they had in stock. Who was going to buy a used 2, 3, or 4 year old C4 for $30,000 when you could get a left over brand new 2004 C5 a dealer was dumping out at only $34,000 or $35,000.
Keep in mind that other "limited" cars such as the C4 ZR-1's have not really been hit by this downward spiral of prices like the "regular" C4's.
As bad as I feel for the current owners of C4's who are selling their cars and not getting the pricing they expected, when I need to get a C4 for my wife in the Spring to go along with our '65 C2 it will be nice to not have to pay a huge fortune for the car. At the very least, by paying the same amount of money than as i might have expected to pay 6 or 12 months ago I will be able to get a later model year and lower milage car in better condition.
On the other hand, C1's, C2's and C3's seem to keep going up in value!
Oh, another factor to keep in mind when considering the values of the different generations. They produced about as many C4's or C5's in one MONTH as they did for total YEARS production on the earlier gneration cars!