As PP's have said, you can do either: allow them to pick one or pick for them. Some CMs seem to enjoy getting to choose, while others don't seem to care. The only time that it's probably better if you pick the pin you want to give them is if you're trying to trade with a CM during a really busy time. For example, I was in line to buy something and the CM at the register had a neat pin, but there was a line of guests behind me. So we did a quick trade at the end of the purchase, but it only took a few seconds because I had the pin to trade already in hand ready to go.