You can certainly start pin trading FOR your daughter at this age, if you want. At 17 months, she isn't really going to be able to do the trading herself, obviously. The pins will just be colorful things she might want to try stuffing into her mouth at that age!
You can also just start a collection for her by buying a few pins that have meaning for your trip - like one with a current favorite character at 17 months, one with the year, one from your resort, etc... Display them somewhere at home, and when she's older and you return, she can add to her collection by trading then.
As for the ebay pin lots, my understanding is that, if you're getting a whole bunch of pins for the cost of one or two in the parks, you aren't likely to be getting "real" ones - they're most likely scrappers/fakes. There is a starter set or two available through the
Disney store - you can look at their website, or go into a local Disney store to order them. If you want real pins, you should probably buy from the source. That isn't to say that it's impossible to get real ones on the cheap from a re-seller who's looking to offload a collection. But I wouldn't place any bets on it...