[ajuc]

If only you 2 are doing this, this won't affect GDP much.

If everybody is doing this, prices rise aproprietely, and GDP corrected for inflation is still a good indicator.

EDIT: BTW there's a reason people don't generally sell cheap things for billions of $ - you'll both have to pay tax from that sale :)

And even if we assume no inflation - government can now invest these money, fueling economy, so this sale had at least some positive effect on economy.