how to resolve this ?

my son recently purchased an item 


he picked up and paid cash for item 


he marked his account as paid left feedback and received feedback from seller and thought all was good



the other day he sees and unpaid item dispute for this purchase


he has sent seller an email but got no response


now the dispute has been closed and he has a strike against him



how do we go about sorting this out as he has done nothing wrong 



of course there was no receipt given for the transaction as my son said the seller was very pleasant and easy to deal with and now this ?? 

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