MattMatts-Momma,
Here are my tips (for what they are worth). I am new to the pin trading also, but luckily discovered in time to be able to be picky about the pins I've bought. So here's my 2cents. Don't know if it will help or is even the best ideas.
1) Go to ebay when you have time to be on the computer for a while and do a search for disney pins. Don't look at ANY ending within 30 - 45 minutes. Skip ahead to at least 30 minutes to give you time to browse.
2) Begin looking at those pins. I right click the link to open an auction and choose open in new window/tab so that I can keep my original screen open, but that's just a preference.
3) When you find a pin that you think is cheap enough (by whatever standards you want, I differ mine depending on how long it's been since payday.

), mark it to watch and then look at what else the seller has. There's a link in the top right hand corner of the listing that says, "View seller's other items"
4) Pick you out a few favorite sellers that have auctions ending soon and try to buy just from them for that particular sit down time. You may find an occasional great deal as a one purchase type thing, but I've found that the way I've been able to get the best deals is buying a few from the same person within the same period. Almost everyone will combine shipping (look for this in the posting) and so the deal gets better.
**Be sure to not checkout for one pin if you do this, contact the seller for a combined invoice. Lots will not go back and give you the reduced rate if you checkout for an individul item.
**If the seller has some others that you are interested in that end in a few days, contact them (before paying also). Many will let you combine shipping for 3 - 7 days. That gives you time to gather a lot from one person.
**Another thing I do is when I find what looks like a seller I like, I made sure to save the seller to my favorites (I do this for anything on ebay) that way I will remember and check them each time I'm looking for something similiar. I know have about 5 sellers of disney stuff on my favorite sellers/store list.
**Remember these are for TRADING. I find that the not so cute ones go for cheaper and since I want my daugther to learn to trade, these are better than cuter ones of characters she likes. I know her; she'll keep those not trade them.
Good luck and hope this helps. I know that this is the system I've used since I started making these plans in February and I've gotten about 15 - 20 for my daughter now, and I don't think I've paid more than about 4 dollars max for any of the pins after you take the total that I spent with one seller and divide by the number of pins. (well except for two that I bought for her to keep that were just special ones, like a bday one)