Potential scam? Seller asking for passport and national insurance details before posting!

I have just bought and paid for a laptop for an auction I won.  The seller is asking for my passport and national insurance details before they post the item.  They are stating that the reason for this is because I am based in Northern Ireland they need to provide a customs declaration.  Any insights into what may be going on here?  Or any suggestions on how to proceed?

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it woud be helpful to see the item number, to determine if this is a scam or just an Ignorant Seller.
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