You never know what a coin is worth until it is sold.
Auctions .. LCS's ...
ebay. You can follow prices and
see a wide range of prices.
When deals like that come along, buy them at what the
seller wants, maybe a little over. Do not let the coins
you want go to someone else.
Then sell the coins you don't want. You can then go back and give the original person more money, after you really know what you can get for the coins. If the person was happy getting what they ask for the coins .. think how happy they would be when you came back and gave them more.
But ...control the deal.