I read this in the early 2000s. It was checked out from either my school or local library (in Canada, if that makes a difference). It was definitely written in English. It may have been part of a collection of short horror stories. It was most likely written in the 70s, 80s, or 90s (although the 90s is less likely). Based on the way the characters spoke to each other and the setting described, it was set during the 70s or 80s.
The shop was one with a bell above the door to alert the owner of new customers arriving. At the beginning, the store was empty save for the owner and a single customer asking about a vintage mirror with unusual border detailing. It was heavily implied that it carried dark memories or contained some kind of dark magic within. Before the transaction was finalized, a couple entered the store and interrupted them.
The boyfriend was looking for a birthday present for his girlfriend; once she saw the mirror, she was immediately drawn to it and started gushing to her boyfriend about how much of a perfect present it would be. Once she heard about its backstory, she quickly changed her mind and her attitude. She was crying and whining to her boyfriend that she didn’t want the mirror anymore, that it was ugly, and how her birthday was ruined.
The first customer did end up buying the mirror and brought it home, but I can't remember what happened after that. I think he may have bought it because something or someone he loved was trapped inside and he wanted to find them, but I can't be sure.