Get the catalogue of the auction house of your choice for free or against small cost participation. Then you'll have the possibility to make offers for missing stamps, rare values or entire collections.

You should however make realistic offers corresponding to real market conditions if you want to get the chance to acquire a missing stamp. Don't forget: the lots of the auctions are sold to the highest bidder and your realistic offer only avoids disappointment in the case your stamp is sold to others.

Usually your stamp will cost you less than your highest bid for it, since the respectable auctioneer increases strictly "interest-protecting".

Interest protecting means: the lots of the auctions are sold to the highest bidder but at the standard advance over the second highest bid.

For the great collector with unlimited budget or for small purses: Auctions offer, nowadays, for each philatelist an immense selection of all kind of collectibles: You should give it a try!