I don't think he can leave feedback unless item is paid. Could be wrong. If he does leave negative feedback then open a non-payer case and if he doesn't pay (again) the feedback gets removed by boots.
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"Mark 11.24 Whatsoever ye desire, believe that ye have it, and it is yours"
I agree with Yankee.
Just to be on the safe side you can contact the buyer and ask him to send you proof of payment if he claims he did actually paid - make sure nothing got lost on either side.
Other than that it is up to the buyer to prove he sent you payment.
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AGED SUPER HIGH LIMIT ACCOUNTS: 10,000/$1,000,000 and 25,000/$500,000