It involves making snap judgments about the appearance of members of the same sex, e.g. clothes, hair style, build, accessories etc. The identity shopper then copies the appearance of people they like the look of, and so feel much better about themselves. Until they see someone better looking, then they copy them, and so on.
Over time, this cycle raises the standards of dress for a large part of the population which is why people in cities always look so much hotter.
Guy: "Thanks! I just try to copy all the hot guys I see walking around."
Girl: "Dude, you are so gay."
Guy: "Nah, it's just identity shopping."