Primarily I buy sets from
ShopDisney for traders when they're having a sale. Broken down, $5 - $7 a pin is what you should expect to pay. Anything less or more than that and you're paying for that pin specifically or likely not getting an authentic pin from a third seller.
Magic wise, since you're buying pins that other people will trade for, keeping the person in mind who is going to trade for your pin (and likely love your pin) is always something I mention. During one trip I accidently traded a CM for a pin a little one behind me wanted. I turned around and offered to trade her. Her mom gifted me a Pixie Dust Pin. It is literally my most treasured pin I own. Pin trading is a lot of fun for a lot of people, so I try to buy pins that will make someone's day.
While they're not advertised as trading pins,
Loungefly pins are tradeable as they're licensed Disney pins. A few are even (IMO) better designed than some ShopDisney pins (That Pooh Disney Character Food Mystery Pin amiright) and are generally lower priced. I bought my first few not that long ago and quality wise, I had no complaints. I still prefer ShopDisney pins but that's just because they come with Mickey backs and Loungefly pins don't. Generally however, I buy at the park, or ShopDisney.
Currently pin trading at WDW involves pin trading stations and CM lanyards have been discontinued for the foreseeable future. Mouselets has a list of the stations if you would like to schedule stops in ahead of time. Prior to my trips, Mouselets tends to always be my go-to for figuring out my trading route.
