Mermaidsandcats, pieces whose value increase over time are hard to find in the fashion world. Generally, they are accessories or limited edition pieces, or once-coveted pieces that are no longer available.
Some pieces that have increased in value are the Hermes Birkin, Jil Sander clothing from the Jil Sander era, and Cloak pieces from Alexander Plokhov's time at the helm. I think past season Carol Christian Poell (CCP) pieces have been selling at more than retail price as well.