You should also realize that prior to the FA-10 (i.e., 1999) the blue cards were just "filed" in boxes. Like some of the crap you and I have "filed" in our basements, they were frequently mildewed, water-logged, covered with rat shit and just plain misplaced. There was some minor effort to enter them into the database, but there's a very good chance than anything you purchased prior to 1999 isn't in the database.
That being said, I'm not aware of any law or regulation that requires that you provide a copy of the transfer registration in order to sell it to a dealer. I'll bet that this is just one dealer's way of atempting to ensure that he'll be able to sell the gun to an in-state customer. You can always sell to a licensed individual in-state or the dealer can always sell to an out-of-state customer (through an out-of-state FFL, of course). Some dealers may also be willing to accept your written statement that you legally possessed the firearm in-state prior to the required date. If you purchased the firearm from a dealer that's still in business, they'll still have the 4473 in their files which would document the purchase.
Ken