Let me modify CostumeDude's step by step guide a little:
Step One: Work Really Hard
(for some money to spend on screen used collectibles
Step Two: Save Up Some Dough
Step Three: Let People Know
what you want to buy but not how much you have to spend. Meanwhile do your homework on potential sources: established professional prop sellers, auctions, other collectors, etc.Step Four: Someone Offers You Something
Step Five: Decide what you think of their price. Counteroffer if you think the price is too high or go ahead and pay if the price seems fair.
Step Six: Either receive the item or continue negotiations
Step Seven: Don't be bullied into a price you think is too high - walk away (I bet this is where Costumedude gets stuck)
Step Eight: Buy at a fair price when the time and opportunity is right. I bought one of my most desired pieces at
80% off the price the item was originally offered because I waited 3 years and kept negotiating. When the negotiations for a private sale to me cratered and enough time passed, the seller finally auctioned off the piece, it was less popular, the economy wasn't as good as it was when I first saw the piece, and my competition was only other sellers looking to resell immediately at a huge profit when there was no general interest in the item.
Try not to buy when an item is hot -- ie when the whole world is thinking about and in love with the film/tv series/actor/actress/genre. Delayed gratification can be your best tool in acquiring -- an example: Debbie Reynolds sure got a lot of Planet Hollywoods best stuff on the cheap (cheaper than what PH paid) when they went bust and dumped their stuff in 2003. They spent millions, she spent only tens of thousands. Patience will serve you well -- if it's gathering dust on the dealer's shelf or your fellow fans are out of love with the film/tv series/actor/actress/genre but he/she/it still strikes your fancy, that's the time to buy!
Golden Rule: Buy what you are TOTALLY in love with, not what is cheap or resellable. In this hobby, you only go wrong if you commit youself to buying things you would easily part with.