The seller will hopefully have retained a proof of postage when he mailed the batteries. You need first to email him as Eddie suggests, as if the package doesn't arrive (and one of my eBay sales took over 10 days to arrive recently) then the onus is on him to put a claim in to the Royal Mail for non-delivery. Any reputable seller will do that and then refund the money to you. The other thing is that if it's not resolved in the next few days, you need to report it to eBay. If you paid by PayPal you can get redress through them too - they will investigate and may refund your money then reclaim it from the seller. When you contact the seller at first, just say that the batteries haven't arrived, when did he post them, and then when he replies if it's been more than a couple of weeks he can go to the PO and get a form and start the claim process off. Any difficulties, keep eBay informed, they have an interest in protecting buyer and seller so they will certainly try to help.