eBay seller sent extra item - what should you do?

[tr0psn4j]

@BlackKnight

Update -

The seller accepted my offer and thanked me for my honesty.

I hosted a Zendikar booster draft on Friday, so I opened one of the packs then. Since I know there are geeks on this board who might be curious, this is what I got in my crappy, crappy Legends booster pack:

Blazing Effigy
Tundra Wolves
Cyclopean Mummy
Devouring Deep
Emerald Dragonfly
Alabaster Potion
Rust
Energy Tap
Giant Slug
Wall of Earth
Blood Lust
Wolverine Pack
Storm Seeker
Red Mana Battery
Gosta Dirk

Nothing great. But it was a lot of fun opening a pack that old.

Not a single playable card! You should have kept that one sealed and opened the other one.