I also take all of my pins (both the traders and the ones I've bought) as carry-on. As others have said, almost every time, the TSA security agent has had to take closer look inside that bag (probably because it looks like a huge mass of metal). I would strongly encourage you to put all of the pins in ziploc baggies, so that when they do inspect them, they're less likely to get lost or fall off the lanyard. Other than having to go through the extra security step, I haven't had any problems taking the pins as carry-on.
In terms of trading at DLR...yes, they do pin trading. As mentioned, check at the cash registers because many of the shops have pin boards. Trading with CMs is a bit more hit or miss...I've seen lots of CMs wearing lanyards on some trips, and very few wearing them on other trips. As with WDW, the best selection is usually best in the morning. Unfortunately, like WDW, there does seem to be fair amount of scrappers showing up on CM lanyards and pin boards.
