You missed it, DCPatriot. The slave seller says, "No sale," and then slits her wrist. You see her walk off, lean for strength against a wall, blood starting to pool at her feet. One of the more subtle murders in the movie, but quite pertinent to Gibson's beliefs about the value of life, even the lives of those who are too old to be useful.
I thought he simply cut the rope from her wrists.
I can't understand it....I wasn't looking at my $20 popcorn or $6 diet coke either. ;^)