The red dot sticker on an artist/price card indicates that the art piece has been sold. This is an American gallery tradition. During scheduled art exhibitions, art is sold and marked with the red dots, but remains on display until the closing of the exhibit, even though ownership has passed to the buyer(s).
This practice is quite unlike typical retail sales policies where the buyer takes the merchandise immediately, because fine art is not typical merchandise in several ways. Prior to any exhibit, there is a contractual agreement between artist and the gallery which requires 1) that the artist provide artwork to the gallery and 2) that the gallery provide viewing space for a specific time period, or the exhibition rights.
Exhibition rights are not transferred by the sale of the art and neither are other rights and privileges such as copyright (the legal right to reproduce the art) or the permission to alter and/or destroy the art. This system is something like purchasing a home lot, where the buyer does not automatically acquire other property rights such as mineral rights, water rights, airspace, etc., which are separate rights in real estate
Collectors are patrons of the arts and willingly postpone delivery of their purchases to maintain the integrity of exhibitions which are meant to be enjoyed by the public at large, an essential motive of the serious artist.
– Mike Dominguez