Personally I think at $20000 he's probably undervalued his collection. He had 2 rooms with TF's (the room shown had the most) and over 1000 toys. That works out $20 per toy at least. Granted that some of the TF's you wouldn't get $20 there's a tonne of them that you'd EASILY get over $20 for. Balancing that out.
Even IF the dealer gave the bloke $18000 or $20000 there's still plenty of room there to make money from the dealers side. The dealer's comment about $5000 worth of labour sounds pretty off though. I reckon the guy who owns the collection would have all the little weapons sorted for each transformer so there would be very little sorting on that end. Perhaps having to pack and unpack them, handling and getting them ready for sale might be worth $2000 tops, but really the dealer was just being a business man. Offer a lowball, try to pay as little as possible to try and make as much as possible. He even knew that his offer of $5000 was an insult/ slap in the face but he still said it. It was pretty clear from his body language that he (the buyer) didn't particularly want to buy or deal with the toys.

BTW If I was offered a Fortmax with box for $1000 I'd take it