Buying and Selling
If you are interested in BUYING a propeller, the following links might be helpful:
Sensenich Propellers:  They do have some new, but non airworthy display props for sale.   These are "modern" designs.
Aeroclocks - Bob Gardner in England  has a number of WW1 era propellers for sale, with frequent additions from time to time.  To my knowledge this is the only active place to look for a selection of WW1 era propellers.
EBay - Always a few, usually modern, props available.   Search here for just wooden propellers, or use this search option to include more results, including other types of propellers (including marine, etc.)
WagAero frequently has display propellers ("modern" type) for around $400.
TAircraft - Jeremy Young has some propellers as part of his grandfather's estate
Propeller "Value"
      Nearly everyone wants to know what their propeller is worth.  The market is sufficiently small and variable that it can be very difficult, if not impossible to determine an accurate number.  Usually the most current and accurate estimates can be made by searching on the "completed listings" option on ebay, using the search query cited above and looking for similar props that have sold in the past few weeks.  Be sure to confirm that the price listed was actually bid.  Click here to perform that search as "wooden propeller".  (You need to be registered on eBay for this.)

In general these factors control value determinations:
Rarity and Provenance - As with any collectible, commonly available items have less value and more consistent sale prices.  Conclusive identification of the aircraft type can often be made by referencing the stamped information on the hub or blade, but frequently either just the engine can be determined or,  in many cases, no definite usage can be determined.  The actual usage of a given propeller is rarely known, so beware of claims that a particular propeller was used by a specific pilot or in a specific event. 
Condition - original condition improves value considerably.  Damage may not necessarily reduce value to a great extent, but attemps at "restoration" typically do reduce values, often considerably.  Decals are an important component of many wooden propellers, although not all manufacturers used them.  They, too, add value.
3.  Aesthetics - some propellers are basically ugly.
Ability to Ship - a 4 blade propeller, or a very large (10 feet or more) propeller can make the cost of  packing and shipping it almost as much as the value of the prop.
5.  Airworthiness - this is rarely a factor, as most originals are used as display props, and new replacement propellers are available or can be made for flying aircraft.
Posted 2/28/05
CAUTION: -- EBAY SCAM ALERT  --  On the recent sale of several propellers on eBay, a number of underbidders have been contacted offering the item for sale under the claim that the high bidder did not complete the transaction.  THESE ARE COMPLETE SCAMS.  Contact the original seller directly if you are offered this option.
Our new Wooden Propeller Forum - you can post your interest on the discussion board or check to see if anyone has posted a propeller for sale on the same board.
November, 2006 - I have finally photographed and added descriptions of some of the duplicate propellers in my collection and put them up for sale.  Please click here for Early Wooden Propellers For Sale.