Used, second hand, or vintage; they are all different terms for the same thing, and there are several types of retail entities that sell them. Privately owned vintage shops or boutiques, thrift or charity stores, and consignment are the major places to buy vintage, but there are a few differences between them.
A vintage shop/boutique is a privately owned store in which all of the items for sale were purchased by the store and are being resold at some sort of premium. The store can get their merchandise from any number of sources, but the most common source is a rag house
through which they can purchase large bails of clothing. Other sources of stock include swap meets, vintage fairs, and individual hunting at estate sales or flea markets.
Thrift and charity stores sell used clothes that were donated by individuals. Depending on the individual store, profits from the sales that go directly to charity vary, but all sales contribute in some way monetarily to the charity who owns the thrift.
Consignment shops are places where individuals can sell their used clothing through a dealer. The shop, as the dealer, takes a percentage of the profit for each item sold. If items do not sell during the period they are on the sales floor, they are returned to you and you receive no compensation.