When a big budget movie goes into production, the wardrobe stylist / costumer is given a budget for clothing for the film. They may elect to pull clothing from a studio's collection (paying a rental fee), or purchase name brand clothing from stores to dress their stars. The clothes are then altered to the actor/actresses size for filming. After filming finishes, and there is no possibility of reshoots, the clothing is either returned to the studio wardrobe department, or sold. There are a number of stores in Los Angeles that specialize in used studio wardrobe ie itsawrap.com that will offer a higher price, and then resell it at a higher price given its provenance as being in the movies.

If a star likes a particular piece of clothing, they can general buy it from the wardrobe department at a discount.