OK, this is long (sorry)
Yes, if pictures listed in the add are quite a bit darker than the eggs you received, the you have an issue of "Item not as described" or pictured.
I would just go the seller and tell them that you were expecting much darker eggs than this. That the eggs pictured are much darker and that you are unhappy and would like your money back and ask what the seller's policy is to refund.
If after you have tried to negotiate with him in good faith and he doesn't respond reasonably, then you need to pursue the process on e-bay (I have never used it but there is one) and of course, if nothing works then you post the dissatisfied response under the feedback section for this seller. And, it is important to list in that comment exactly what you got from him so that other buyers will avoid this seller. Make sure you don't wait too long to do that as there is a window of time when you can respond with feedback.
But I would give the seller a chance first to respond to your request for a refund and see what he will do for you. You have a lot more power to effect this person's reputation and business than you realize you do. I think most sellers are well aware of this and want to make sure people are satisfied.
Oh and one more thing. If the seller offers to provide a partial refund like the cost of the eggs but not the shipping, if I were you, I would meet him half way and take that and here is why- If you did not know this seller or did not have lots of recommendations from other people, then you took a little chance and there's nothing wrong with that but it carries the risk that you might not be satisfied.
I bought my eggs on e-bay and I am very happy with my birds, but before I bid I got recommendations from 2 people here on BYC, 2 people from the American Marans Club and two big named breeders who all said the seller had nice birds and eggs.
Good luck!!!!!