Assuming I read the initial post correctly, it sounds like the buyer wants to return those that are not Popeye corks and get reimbursed for the ones that are being returned. My fear would be that your math and his math might be based on a different formula. I'm not sure what you or others here have in mind on a partial refund, but I would almost think/expect that, since about 1/5th of the marbles are being returned, s/he'll be looking for a 20% refund.
Personally, I would almost suggest that you have them all shipped back and give a full refund. My thinking/reasoning for that suggestion is that there may have been other potential bidders who noticed these odd ones and didn't bid or quit bidding earlier than they otherwise would have because of the "dead weight" they didn't really want. If the two types had originally been separate out and only the true Popeye corks made available, it may be that the final bid would've been higher, when averaged out per marble. (But then again, maybe I'm off in my thinking on that one.)
Based on the one photo I saw above, I would think the size and quality of your photos will allow you to confirm that what you sent is what you get back, should you go the return route -- partial or full return.
Holly