Yeah, unfortunately, if you plan on lanyard trading, chances are good that even if YOU get cheap authentic pins to trade to cast members, you'll actually end up taking home scrappers. Sometimes it's easy to tell the bad ones, so you can avoid some of them. But sometimes it's tough to tell.
If know and accept that, and still want to lanyard trade (especially when kids are concerned, they just want to do the fun thing, not worry about scrappers), then the best cheap traders are usually to find a Disney outlet. Sometimes you can find $1-3 pins. Next best options is to see if someone in this thread can recommend a seller. Some of us that used to be around during SALSA sales and the old cast sales can sell pins for about $3 per pin, maybe better.
If you're talking about better traders (like, with backing cards) for pin events and trading days, then I'd look at collections or large lots. It usually takes a while to get a good selection of traders going. Some people find it hard to start a big trading pool, because it's hard to buy traders when you could spend the money on your keeper pins, right?
Big red flags for buying pins and pin lots off of eBay:
-Avoid sellers from China/Hong Kong
-Avoid sellers who use "stock" images of lots, and have a lot of lot sales.
-Feedback is not necessarily an indication of a good seller- a lot of people leave positive feedback because they don't know about scrappers, or don't care.
Asking in a place like this is the best way to find good sellers, so good luck!
If all else fails, let me know- I have some I could sell you (things like purchase with purchase pins, and some old open editions I picked up some outlets and SALSA sales), for around $3 each. Just let me know if you need me.