Hopefully you opened a "significantly not as described" case.
Under normal circumstances, since the Seller sent you an item other than what was described in the listing, the Seller will have to pay the return shipping. If the Seller and you are both in the USA, the usual practice is for eBay to send you a prepaid shipping label; you pack the wrong MacBook carefully back into the box it came in, apply label, and send it on its way. Using the eBay label automatically keeps the return tracking info associated with the original purchase so when it gets back to the Seller, eBay will automatically know that and will refund you shortly thereafter.
In a case like this, the Seller might want to tell you to do an exchange, and I think most regulars here would agree that it is NOT a good idea to get involved with the Seller promising to send you the "right" MacBook if you return the "wrong" one. For most people, it's best to Keep It Simple, just do the eBay return and let that come to a conclusion. Then if you still want to deal with this Seller for the "right" MacBook, make that a completely different and separate sale.
