I work in the auction world and do my own share of valuations of vintage clothing on a weekly basis. It's a tricky business because auction prices vary and there are many factors that can make something valuable. My advice is to always consult a repuatable person. Lately I've received a few emails from people asking me general advice on who they should go to and how auctions work. A lot of vintage dealers seem to be giving advice on values which is not always correct and they are putting their reputation on the line with their lack of experience.
For a start, consult someone that is in the auction business if you are interested in having your items considered for auction provide clear photos of your items including the label, note any damages or imperfections also include any relevant historical facts about the piece for a start.
Now for fun and to help you get started go to
search Mariyn Monroe and you'll be able to access a video and short quiz where
you'll be able to guess how much Marilyn Monroe's dress from Some Like it Hot is worth.