It's virtually impossible that a high volume seller of Disney pins on eBay is selling legitimate stuff - nobody has a 500,000 pin collection that they are "looking to sell off". A 300 Disney pin collection is a very big collection - I know, I've got one, and every one was purchased at a Disney park, Disney owned outlet, or
Disney Store On line, before being traded at the park.
But really, Cast Members have no good way to tell if a pin is a fake, so they will trade for anything that isn't ridiculous looking, which does hurt the value of pin trading for those that try not to encourage theft of intellectual property.
Having said that, I am quite sure that some of the pins in my collection now are probably fakes, since I could have picked them up while trading with cast members. Since our family just does it for fun and we have no intention of trying to make a profit off of it, I don't much care if Disney doesn't care that they are fake. I just look for pins that I think are cute.
It also is possible to trade with visitors. It happened to me this summer. A Dad came up to me and said "Excuse me, I know you're not a cast member so don't have to trade, but my daughter would like to trade with you." I said "Make me an offer" and we made a deal (luckily she wanted a pin I was happy to give up - I would have accepted anything in trade from her, but I am quite attached to some of my own pins, so I didn't care at all what she was offering, although it would have been tough if she asked for one I really liked - and my kids would have never allowed me to say no to her no matter what she asked for).
So how can you save money while still buying only from Disney? If you hit the outlet stores (we stopped at the one in St. Augustine on the way down) you can usually pick up pins in sets for around $2.00 per pin. There are a couple outlet stores in Orlando too.
On Disney Store On Line, earlier this year, they had a clearance in a 3-pin set from "Up" for $6.97 per set ($2.33 per pin) - I bought 20 of those sets (60 pins).
Also, while we were down there this summer, they had a promotion that expired in the parks before we arrived. They had a lanyard with 8 pins that they sold for $15 - before we arrived, the promotion was you had to spend $40 to be able to buy the set - but they had switched to Mickey watches as the new promo. So the old inventory was being sold off for $15 per set - with no requirement for any additional purchase. I bought 15 sets (120 pins).
(Yes, I do have a problem. And no, I have no intention of trying to fix it).
Last year, we were down there and had only a small number of pins to trade, and I had the fever, so I bought some 7-pin sets at full retail price for $30, coming to about $4.50 per pin - that's the cheapest you'll be able to get on site without finding some kind of deal.
We all have a ball doing it. I know many people would consider us nuts for doing that - but so what?
Back home, we have a bulletin board covered with cloth that we use to display the pins, so we do have a use for them once the vacation is over.
Have fun!