Some countries, including the UK and Sweden, allow anybody to buy a domain name under their country code. For other countries, e.g. Australia, you need to have a company in that country in order to buy a domain there.
Booksplea.se have tried to be "smart" in using a ".se" domain because their name happens to end in "se". Unfortunately this has backfired in your case as it has made you trust them less. I would apply the usual checks and balances but not let the se domain worry you too much.