It's easy to overpay for a collection that includes a good sampling of keys. That's why it's imperative for dealers to be conservative in their estimates of both value and grade. The problem for dealers is that they know if they're too conservative that they run the risk of the seller taking a pass on their offer. It's more an art than a science at this point. So being honest and up front with the seller is always the best approach. Instead of saying, "I'm paying top dollar for your books" it's better to say, "You've got some great books here and any comics dealer worth his salt will want them. Here's the thing, your books are going to sell for the same price no matter who sells them. So legitimate dealers, as a whole, will all pay close to the same price for your collection. My offer is strong and I've got cash to pay right now."
Most sellers will understand that approach. it works for me.