What do they do to fancy costumes used in movies after use?

The production company owns most of the costumes unless it is something that the actors bring in. All costumes are retained by the production company. Some are later auctioned off and some generic costumes are sold, rented or reused.

In fact most studio have a division that rents out costumes and props. It is a secondary source of money. They have large warehouses of this stuff. For eg: here is a link to Warner Bros costume department.

In addition to the answer given by @BobbyAlexander , I have seen them on display at theaters in Los Angeles, and on traveling displays regarding the movie. For example, I saw a traveling Star Wars collection that included a Darth Vadar costume, stormtrooper costumes, even Yoda. It also included other props from the movie such as light sabers, droids, and miniatures models (X-wings, Y-wings, tie fighters, etc.). They were set up as if they were a museum exhibit - miniatures on pedestals, and larger pieces behind red ropes or glass, with printed descriptions next to each piece. (It was pretty cool!!)

Also, for movies & shows with more "ordinary" clothes, the actors sometimes get to keep the clothes they wore. For example, Sarah Jessica Parker claims that she "kept about 70 percent of her Sex and the City wardrobe after the show." article on people.com

In addition to Bobby Alexander's and BrettFromLA's answers, I would like to add that sometimes costumes are put to an auction for fund rising and charity purposes. Sometimes, actors may buy the costumes if they're interested. And as per your question, costumes in the movies like star wars, Iron man etc are put in the online stores for sale. Check these sites, propstore, propworx.