1. That NPD survey is just a survey, not actual data like sales data they collect.
2. The trade-in question can all be dependent on the wording used in the survey questions. I don't trade my games in to gamestop either for example, because it's a ripoff. Go on ebay/amazon marketplace/craigslist and see how many people are selling their used games, it's a lot. I wonder if they asked if people lend each other games, and what the answer to that would be.
3. If you kill the used market, you'll have more piracy.
Well, that's probably what they can do for now, because NPD does not have access to sales data of those digital game web sites like steamworks. They may be able to get them eventually, but apparently not yet, that's why they have to rely on surveys. Nevertheless, a survey, if done properly, is (much) better than nothing.
And regarding to the trade-in question, IMHO, if second hand game market is dead, those who buy (or sell) second hand games are not going to buy the same amount of games they did before. If they can't resell their games they are more likely to just pirate or simply buy less games if piracy is not an option.
Technically, there is no reason why a digital purchase can't be transfered to another person. However, the business reason is very simple: there is no real difference between a new game vs a used game, if you don't care about the quality of the game media or the manual. For digital purchases it's even more obvious. That's why they want to kill this market, because if most gamers do that, games with less time value would not sell very well.
A more "proper" solution for this is, IMHO, to use rental model. You can buy a game at full price, so you can play for any time length, but you can't resell it. On the other hand, you can "rent" a game at reduced price for, say, a month, a week, or even a day. This is possible with digital downloads because the upfront cost is very small. If you just want to try out a game, you can buy a "day-pass" to try it for a day (which should be very cheap, like one dollar or something). You can buy the full game if you like it and want to play it whenever you want. However, if you think you are just going to play it for maybe a month, you can buy a month-pass for a reduced price.
Blizzard is actually doing this in Taiwan and South Korea for Starcraft II. The full game is NT$1,950 (~US$65). A month-pass costs NT$300, about US$10. A day-pass is NT$50, about US$1.6. They are just trying this out, so I don't know how well it goes, but the publisher in Taiwan seems to be very satisfied with the sales of Starcraft II (although it's possible that most sales are of full games).