Seller trying to scam us on ebay?

So my sister sold her iphone on ebay last month and buyer is responding now that he never received the phone and now he open a case to get a refund(The guy seems like he owns a store or something since he has like 5k stars). Problem is she didn't asked for the tracking option and she mailed it like that. I know she basically screw her self because now the buyer can win any case since there's no proof whatsoever that she sent it except for a receipt. My question is, is the seller trying scam her? I mean the phone could be in his hands, and if is not where is it then! We haven't received it back through mail. What is our options if we have any? Can we send a refund with no phone in our hands? We only have three days to solve this case. Thank you in advance. (No useless answers and no need to reply the obvious just helpful answers)

She sent an Iphone without tracking information? She is indeed an idiot. However, if she has a receipt from the post office that may help. I guarantee he has it.

It is her responsiblity to send anything with a tracking number. That is the only way to guarantee delivery. With the millions of items that get lost or stolen in the mail every year you never send anything of value without a tracking number for any reason. There's no way to know if he is a scammer or not but he will win and get his money back

Newbies selling smartphones on eBay attract scammers like blood attracts sharks, and the result is just as pretty.

Your sister's options are to refund the buyer in full now or wait for eBay to do it. If she waits for eBay to do it, that case will count against her seller performance and she's risking a neg.

Even if the buyer honestly didn't receive it, without DC or insurance USPS has no way to look for it. If the return address is still attached and legible, she may get it back in a month or more.

In any case, why do you care what the buyer's motivation is? Your sister "screw her self" by trying to sell the most-scammed item without the first clue what she was doing.