Disney pins are about the same price on the ship as they are in the parks. You will find "rack" pins, limited editions, captains choice (usually LE1000), artist choice (LE750) and event pins to name a few. Except for special occassions like the Panama Canal Crossing Pins and the annual pin cruise the prices range from $6.50 to $12.50.
I think the cheapest ones come at $6.95. The fancier they get, the more expensive of course. Others run at about $8.50 and $10.95. I got my special edition Antigua pin and the special edition West Coast pin for $10.95 each.
Yes, that is what many folks do. Although the pins dollar for dollar on the cast members lanyard will probably be the same you will have the opportunity to trade for DCL pins.
It is unusual to find a LE (Limited Edition) pin so don't buy on ebay expecting to be able to trade for a pin of higher value, but if you need a supply of "traders" for the cruise then ebay is a good way to go.
Some folks on ebay sell pins in "sets" or "lots" so it is a good idea to search using those keywords. You may be able to get all your traders from one seller in one transaction.
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