Even if these are part of the known population you should not be paying top dollar for a quantity of them. If he wants to sell them one at a time he might be able to get $40 each but if you are going to buy his whole supply they should go for a wholsesale price.

Obviously you will not be able to determine whether these are part of the known supply or an unknown hoard. Once you explained to him that the value would be reduced by a large quantity you can't tell if he is being truthful about the number he has.

Rather than explain about why you need to know how many there are, in the future if they ask you why you want to know, tell them you want to be able to make an offer on the whole lot.