The aliens came bearing gifts. After giving away some free samples they introduced their product lines.
There were energy generators that used no fuel and space ships and flying cars. Credit was extended for the asking to those with sufficient collateral.
The governments of the world were the biggest borrowers. They took out huge loans to ensure that they possessed the best alien weapons.
When the people of Earth were unable to make their payments on time the aliens had no choice but to proceed with foreclosure.
The new inhabitants of Earth got a super deal on a fixer-upper world.