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"buyer agreed to post cost when purchasing"

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Yes, the buyer agreed to the cost because they TRUSTED the seller to be honest

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There was no mention of “handling” charges, so the buyer is reasonable to expect that the cost quoted is the real cost

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"negs are for scammers"

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Yes, again you are right, but when the OP gets the answer

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"she had more urgent things to think about than refunding me"

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That screams SCAMMER to me

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NEG her … stand up for the honest sellers on eBay