There were many things Ismene was, to name a few; cold, rigid, and extraordinarily sinister. Seedy was not one of them, she had her ways of dabbling in despicable human behavior, but traveling down the dirtier cobblestone pathways of Knockturn Alley was not something she made a habit of. There were occasions when it was absolutely necessary, but her family enjoyed their high standing far too much for it to be associated with here.
However, as it did specialize in old, dark magic it was the place to look for texts and relics concerning marriage bindings. Especially those who killed the bounded when the conditions were not met. Having Molly there to help with the research was a plus, but she was not going to leave the fate of her personal life entirely up to Weasley.
She knew most people's faces and names, especially those who made daily appearances in wizarding society. It was her job to, and she experienced small pleasure when she disconcerted people. That is why, when she stepped out of Borgin and Burkes into the slightly chilled air, she took note of the man staring at what seemed to be a display of shrunken heads and didn't refrain from speaking. "I wouldn't consider purchasing them, Mr. Pepper, they're locally made."