Well he hasn't actually been honest about the listing has he? How do I know its only 9 weeks just on a random strangers word?
He would have a receipt/invoice which he hasnt provided and said he wouldn't provide.
Omission of information is not lying. If it was not used for mining and instead used for gaming would you have expected a link to the seller's steam profile so you could analyse exactly which games they played on it and for how long?
If whether the card had been used for mining was of such paramount importance, then you should have asked before buying it.
The lack of receipt/proof of age is a separate issue, but by the sounds of it, it would be better for both of you if you just send it back for the refund


