I've sold my Macbook Air. Panic!

I recently sold my MacBook Air on Ebay. I signed out of iCloud etc & erased the disc twice & I installed the OS twice. The machine's first screen asked "What country are you in" , which I presumed meant the computer was erased & ready for a new owner & that's how it was sold.

After 10 days, the new owner contacted me & said he needed the computer password to get into the hard drive. He knew the prompt.

How could he know the prompt?

Should I give him the password or ask for the machine to be sent back? Could all my previous files still be on the hard disc?

I cannot understand how this has arisen.