If the paper tape on both ends of the shipping carton is still sealed leave them alone. Otherwise I would open them and check. I bought a set that appeared the same and a couple of cars had broken seals and one had been dropped out of the box and damaged and put back in the box and advertised as unopened. I still have that car as a reminder.
Also, matched serial numbered sets are more valuable than mixed serial numbers.
The only sets I've seen sell for high amounts were matched serial number sets with the 2-rail locomotive. The 2-rail locomotive is worth significantly more than the 3-rail version. In equal condition 30-60% more for the locomotive. Which brings us to why there is no 3-rail car passenger car set. For the simple reason that 3-railers see no value in the detail and accuracy of the cars so would not pay for them.
500 sets were produced. Is the 800 number etched on a brass plate? Is there also a Lionel Smithsonian brass plate on the bottom. I'm pretty sure it is not a Lionel serial number. Fine Art Models (who made the sets for Lionel) also produced 500 sets to sell under their name. To the best of my knowledge any serial numbers between 501 and 1000 would be Fine Art Models, not Lionel Smithsonian, although I have not seen numbers that high before.
Jim Seacrest and C.L. Gibson are gone and there are few if any collectors remaining willing to pay premium prices for any high end brass of any scale. It is why the high end market is shrinking so fast. There are a few who think they can resell them at even higher prices because they are supposedly "rare". They're not that rare considering the size of the market willing to pay for them.
I would agree with the $400-500 per car. The sleepers maybe a bit less and the others a maybe bit more. Depends who is in the market at the time and how much of a hurry you are in.