First, check all your pictures for identifying info such as make/model/spec etc and do a quick google to see what specs the laptop YOU had, had. After confirming the specs, if it couldn't have had the lesser processor, provide the info to the buyer. If it could, apologize for the mistake on the parts laptop and offer a refund with return (if I think a claim may be bogus, I ONLY offer return and refund). If he wants it, he won't return it.

In the future, don't use someone else's product info as your own (they could be wrong too). Know and describe what you have. Use other listings to give you an idea of what you have if you don't know, but always describe your particular item. Take lots of pictures. This helps "describe" the product to potential buyers, and helps you if you get a return that was tampered with or otherwise not in the same condition as when you sent it. For me, old and parts electronics aren't worth the hassle to sell on ebay, because I put a lot of time into researching what I have, documenting, and listing...a $10 or $20 parts machine isn't worth the involvement.

If you don't want to take the time for such items, describe the item you have to sell, including the info written, stamped, or stickered on the item and some pictures. If you can't verify some information (such as processor) with the info on your item, say so in the description. Quick and easy, and the burden is on the buyer. You will get the occasional return or have to make an occasional refund, but if you do enough volume (and it doesn't take much), then you'll stay on the upside.