I'll add my 2 cents as my DD6 just traded for the first time last week, and we had no idea what we were doing... So I'll tell you what we did and you can decide if it makes sense.
First, we bought a few pins at our local
Disney store about 2 weeks before going. They were on sale, so not too bad. She picked pins that she liked, including some she didn't know (like the ghost from the Tim Burton Holloween movie). She bought doubles of a few, but not many. She got maybe 12 total.
We arrived at the beach club and Art the greeter was her first trade. As others have said, a soft landing helps.
Again, I'm no expert - but the rule seems to be that you can approach a CM and trade (if they have pins) and when trading you can say which one you want to give up from your batch, and which one you want to take from theirs. After all, I don't believe they are really their 'collections,' rather they are just a part of the profitable Disney magic. Econmics aside, you ALWAYS get the benefit of meeting a CM, and that is well worth a buying a pin, to me.
My wife bought a lanyard when we arrived - to hold our park tix and the pins. When it was time to trade, our DD6 took the lanyard. If you are nice and willing to give up any of yuor pins, you could certainly let the CM choose one from your lot.
My daughter quickly decided she wanted princess pins, and was always successful finding them, or something else she thought was cool. We traded one pin with another guest we met - she said the ghost was rare. OK - we weren't going for rare (and they were just on the shelf at our local store) so we traded - and had a nice conversation.
We didn't seek trades but did them when the time seemed right. By the end of our trip, we had traded all our original pins for new ones. Our DD6 liked the original pins, but the new ones each came with a smiling face and a few nice words - at we all figured out what pin trading is all about.
So akuna matada. Pick something, anything your DD6 likes. Vehicles, villians, princesses or pirates - there's plenty of everything. Let her change her mind each time - doesn't matter. Just get a few disney pins and dive in!