Zack, definitely contact the guy you bought it from. This constitutes a violation of ebay seller policy. If he didn't disclose that the entire sprue was missing, you should be able to get your money back by filing a complaint with ebay.
Even if he said something like "Sold as is", this is still an incomplete sale and I would file a claim with ebay. They have an arbiter's board who will look at the sale and make a fair decision.
And you might also find that the seller still has the sprue and just for some reason didn't realize it was out of the box, IN any case, your first move should be to contact the seller.