(CNN) Scott Amos says his mom's been after him to get his childhood junk out of her Reno, Nevada, attic since he moved out on his own more than 20 years ago, and now, his whole family's glad he did.

When he went through one of the boxes, he came across a rare unopened copy of the Nintendo Entertainment System game "Kid Icarus" that has been sold at auction for $9,000, including the buyer's premium.

There are fewer than 10 known sealed copies of the 1987 game.

The shrink-wrapped cartridge was still in a JCPenney bag along with a receipt showing that it was purchased in December 1988, when Amos was 9.

Amos says the game has been the talk of the family since he picked up the boxes for Mother's Day.

"Our only theory is that it was a Christmas present my mom bought for us and never actually gave to us," he said.

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