Barter in Blood

• WWizards of the Coast2012-05-01
  Official Ruling
  If a player controls only one creature, that creature is sacrificed.
• WWizards of the Coast2012-05-01
  Official Ruling
  The active player chooses which creatures will be sacrificed first, then each other player in turn order does the same. Then all creatures are sacrificed simultaneously.
• WWizards of the Coast2012-05-01
  Official Ruling
  Barter in Blood doesn’t target any creatures and may be cast even if a player controls fewer than two creatures.

Rulings via Scryfall