We do the pin trading thing too - though my eldest has declared he's now done and officially bored with it. He started it off in our family about 7 years ago. Then my middle son, then my daughter, then my wife, and I was the holdout... but somehow, with more trips in under my belt then theirs (and a bigger wallet), I have ended up with a larger collection.
For family trips, or any combination of family members more than just me... I ALWAYS get pins on eBay about a month prior to the trip. Depending on who's going determines what lot size I need. THen when they arrive, I divide them up and pass them out. Everyone gets an equal number - but that doesn't mean that they will take all of them with them to trade. IF they see something that they like and want to keep it, they do. The nice thing about eBay is you can find the pins quite cheap - the best deal I ever got was 80 pins for $60. They aren't always easy to snag... but with persistence, you can get them very inexpensively. I would never pay Disney's retail price for 80 pins - though I do like to get one new pin each trip - whether is a dated pin (in the design), or a resort pin. I haven't collected all of the resorts I've stayed at yet, but eventually I will.
On eBay I have only once encountered a problem with fake pins... avoid anything from Europe or Asia. You want to read the descriptions carefully and look for something like "authentic Disney pins". Avoid anything like "CE" or other two letter descriptions like the plague. Those are fake.
In the parks the CM's have two different types of pin holders. There are black ones and green ones. The green ones are FOR KIDS ONLY. Those CM's will not trade with adults. The black ones are for anyone.
A big tip... if you get to the parks early - go to the Guest Services desk. Each park has one. They keep a big book of pins in there - and you are allowed to ask for and look through it. The best pins are found early in the day - these include CM exlusives, limited edition, etc, etc. You are allowed to trade up to 2 pins from the book, per person. Unlike the CM's in the park - they don't limit the pages of the book to one of the same kind of pin per page. Many times I have seen the exact same pin 2 or 3 times on a page. (After you leave, the CM might rearrange the book to spread them out).
ALSO - there are RANDOM CM's who wear their pin holders backwards. They CM's are holding "mystery" pins. They won't let you look at them. You are essentially "gambling" on what you get. The nice thing is though, that if you don't like or want that pin, there's always another CM not that far away who you can re-trade with.
Not all CM's wear pins. But all CM's who do are to trade with guests if they ask. They are limited by no more than one of the same kind on their holder, and the green / black rule mentioned above. Otherwise, trade away!