Don't start!!!
DisneyWood,
I agree with ScottieBoy (and with your baseball card analogy) that this is a very expensive hobby.
From a historical perspective, pins have been given at Disney for many years, mostly with CM's but also occasionally to commemorate park milestones, and like buttons today, were often given away.
Modern (Disney) pin trading was introduced as part of the Millennium Celebration (along with the Epcot tombstones/ LaL). But I don't think it really caught on for about another year. Now of course it is overwhelming...but moreso at WDW than DL.
There are a number of different levels this can be enjoyed? at. The simplest is by trading with CM's that have lanyards. They have pins from guests (sometimes you can get something special traded by an unknowing guest) or what are referred to as cast pins (not sold in stores, but made to populate lanyards for CM pin trading)...not to be confused with CM pins that are only sold to them at Company D and are not traded while on stage.
The rules of engagement are simple here, any one of your pins for one of their's.
Then you can trade with other guests. This gets a little shaky as they aren't necessarily prepared for the advance and proposition as are CM's. More often kids approach others. There are no rules here...you can trade 10 of your's for one of theirs etc.
The most difficult area are the "professional traders" that have their books set up on tables in designated trading areas. You have to know value and what you want to deal with these folks. They are not CM's so etiquette is not always at the forefront.
The other alternative is bidding on Ebay. This is sometimes best if you are looking for something specific. There are also trading websites, one of the most reputable is
www.pinpics.com. But again the rule of engagement are more complex.
As for making money off of this, the only one's I know who consistently make money are those living near the parks that are present for releases of the various limited edition, special event, artist proof and surprise pins that turn right around and sell them on Ebay. A lot of time and effort, but some do well. [Other purists contend that these folks and practices are what is ruining pin trading for others *read amateurs*]
My advice is if you enjoy collecting and like them, fine, but I would not expect it to provide for your retirement.