You are only obligated to offer what you feel is a reasonable price for the chips. No one is forcing the seller to make a deal. If they don't like your offer, they do not have to accept.

Some people feel that in a situation like this, the more knowledgeable party, in the case the buyer, should make the first offer and that the offer should be reasonable. For example if you offer $25, but would be willing to buy for $100, that would not be reasonable.