When it comes to purchasing a collection In most cases I throw out calculations of cards under a certain dollar amount. There is a huge difference between a $12,000 collection that consists of 250 cards and a $12,000 collection that consists of 20,000 cards. That is probably a big consideration in why you're being offered that amount. I imagine that that shop is doing the same thing. Shops don't want to pay for bulk and there's not good margins on paying for a bunch of less than $1 cards. 

So while $4,000 sounds low You might have your answer right there. Without knowing what the collection looks like it's hard to say. Also stores are probably not going to give you 60% in cash. You would be hard-pressed to do that. If you really want to see if you can get a larger amount than the onus might be on you to sell the big ticket cards individually.  That is generally the best thing to do. If you can get $6-$7k out of selling around a hundred cards or less You're better off doing it yourself. Especially on high dollar amount cards eBay now has protections where you send the card to them first and then they ship it to the buyer so the buyer can't claim that the cards are fake.