1) Yes, voluntarily or NPD gives them an incentive.
2) The numbers they report are an aggregate of data sourced from those retailers. That constitutes 90-95% of the USA retail market. The remaining 5-10% is then estimated by NPD using a variety of proprietary and well-developed algorithms.
3) Leakers are NPD clients (they pay money to get NPD data)
4)
Here is a list of USA Electronics retailers that give NPD their sales information:
4Sure.com, AAFES, AAFES.com, ABC Warehouse, ABC Warehouse.com, Abt Electronics, Abt Electronics.com, Amazon.com, American TV, AmericanTV.com, Apple, Apple.com, AT&T, Beach Camera.com, Bernie's8, Best Buy, Best Buy.com, Bjorn's, BJ's Wholesale, Blockbuster Video, Bloomingdale's, Bob & Ron's Worldwide Stereo, Bob & Ron's Worldwide Stereo.com, Boscov's, Boscovs.com, BrandsMart, BrandsMart.com, Calumet.com, Circuit City.com 1, CompUSA.com, CompUSA/Tiger Direct4, Conns Appliance, Conns Appliance.com, Cowboy Maloney's, Crutchfield, Crutchfield.com, Dell.com, Dillard's, Dollar General, Duckwall Alco, Dunham's Sports, eCost.com, Electronics Expo, Electronics Expo.com, Electronics Express, Electronics Express.com, Family Dollar Stores, Flanner's, Fred Meyer, GameStop, GameStop.com, Grupo Famsa, Hastings, hhgregg, hhgregg.com, Home Shopping Network, Home Shopping Network.com, Hunts Photo & Video.com, Huppins, JCPenney Company, JCPenney.com, Ken Crane's10, Kmart, Kmart.com, Kohl's, Kohl's.com, Curacao, Curacao.com, Listen Up, Listen Up.com, MacMall18, MacMall.com18 , Macy's, Macy's.com, Magnolia Hi-Fi, Marines Commissary Exchange, Meijer, Meijer.com, Myer Emco9, National Camera Exchange.com, Navy Exchange, NavyExchange.com, Nebraska Furniture Mart, NewEgg.com, Office Depot, Office Depot.com, OfficeMax, OfficeMax.com, OneCall.com, OnSale/eCOST.com17, Ovation Audio/Video Specialists14, Overstock.com, P.C. Richard & Son, P.C. Richard & Son.com, Pamida, Panasonic.com, Paradyme, Paul's TV, Paul's TV.com, PC Connection Express, Queen City Appliance, QVC, QVC.com, R.C. Willey, R.C. Willey.com, RadioShack, RadioShack.com, Rakuten.com, Reliable.com, Roku.com, Sam's Club, SamsClub.com, Sears, Sears.com, Shopko, Shopko.com, Sixth Avenue Electronics14, Sixth Avenue Electronics.com14, Sprint Store, Staples Third Party Direct, Staples USR, Staples.com, T-Mobile, T-Mobile.com. Target, Target.com, Tech Depot.com, Tekserve, Tekserve.com, The Bon Ton, TigerDirect.com, Toys R Us, Toys R Us.com, Ultimate Electronics12, Vann's, Vann's.com, Verizon, Verizon.com, Video Only, Walgreens, Walmart, Walmart.com
Here is a list of USA Electronics Distributors that gives NPD their sales information (to retailers):
Arrow Enterprise Computing Solutions, ASI, Avnet Technology Solutions, Ingram Micro, Navarre, SED, Synnex, Tech Data, Westcon, WYNIT
And here is a list of USA Direct Marketing Resellers / Contract Stationers for Electronic Software:
Direct Marketing Resellers: Best Buy for Business, Calence (a division of Insight), CDW, Insight, NeweggBusiness, PC Connection, PCM2, PCMG3, Tiger Direct, Zones
Contract Stationers: Corporate Express, Office Depot, OfficeMax, Staples