Silens,
An average beat up, late featured Carbine is worth $500. So you know you did well.
Read someone said "even if import marked" you got a good deal. Doesn't at all mean it is. The seller obviously didn't know carbine values and you got a great deal. Some are marked length wise lightly stamped lightly, some like Blue Sky stamped very hard, some you have to take the gun out of the stock to see the marking elsewhere.
Take it down and see what the markings are on each part. If you don't know what the markings are, buy a book. Cheapest is Riesch I think the 6th edition is out now, about $20 on ebay. No book is 100% correct, but it will tell you what to look for and where.
Then you have $250 + $20 = $270 invested. If it has correct parts to the serial number range, marked correctly also, you will know. Don't know where you live, but maybe someone lives by you that can help out.
Your gun store guy offered $800 remember he wants to make a profit. He would have tried to sell it for more. Now I don't know how much he knows about Carbine's either. But from what you have mentioned it might be worth a bit more then you think. I have seen import marked with late features sell for $800, hard to believe I know.
Pictures of the parts when you break it down would be very helpful in someone giving you a better idea of value.
Or you could just triple your money and have 20 guys emailing you wanting to buy it.
My advice is to find out what you have first, and go from there. It could be a rebuild or it might be a good start to a collection.
Jim