The seller's story sounds like hogwash to me ... but I agree with the others. If it's a lot of trouble to exchange it and the camera checks out 100% OK, you might just keep it. If it's fairly easy to swap it for a new one, I'd swap it.
You might want to check with Nikon to make sure it has not been registered for warranty by someone else. If that was done online the registration coupon would still be in the box, untouched.
Gator Bob Santa Fe New Mexico *D800E *D700 *SB800 *RRS 24L & BH-55 Nikkors: *14-24 *24-70 *70-200 VRII *T-20E III *50 f/1.8 *PC-E 85 *28-300 & Tamron 90mm Macro