What we did this last trip is this. On eBay we got a bunch of pins affordably -- and many of them were "special" pins, cast member or limited ones (hidden mickey ones too), but in bulk they were not expensive per pin (cheaper than buying the cheapest set of starter pins in the parks). We kept the few we really wanted and then traded them with cast members. This meant we could get a lot of cool pins we really wanted that way and spread around the ones other people might really want (rather than getting the cheapest kind), which made the cast members very happy since we weren't deforesting all the rare pins in exchange for very common ones.
David