There is possible ways. For example:

I bought a handphone from you; before you sold to me, the phone is in good working condition, then I change the parts inside etc., then claim that the phone you sold is not working.

Another one is that he can say that item is not received even though you have sent the item, and ask you for the same item to be resent to him again or ask for refund.

Always keep your tracking code when you send item too. You might need it as a proof that you have sent the item you have sold.