Alpha Beta Unlimited
Sales of singles, packs, boxes, and sleeves. All editions available.
http://www.abugames.com/

Blackhawk Games
Site sells virtual singles for use in the Magic online game.
http://www.blackhawkgames.com

CardSorcery.com
US based card dealer selling singles from Alpha to the latest sets, along with boxes and boosters.
http://www.cardsorcery.com

Cargaill's Magic: the Gathering Cards
Individual selling his excess cards. Mostly uncommons, but also stocks rares.
http://cargaill.1hwy.com

CCG-Warehouse
Offering singles as well as sets.
http://www.ccgwarehouse.com

Chargalley.com: Magic
Offering single cards, packs, decks, supplies, and booster boxes.
http://www.magiccardsforsale.com/

Collectandplay.com
Specializing in Magic The Gathering card singles.
http://www.collectandplay.com/

Cutting Edge Cards
UK Magic the Gathering, Harry Potter and Pokemon singles, booster and box sales. Site also features an articles section, links, and downloads.
http://www.cutting-edge-cards.com/

Forstle Card Shop
Magic singles for sale. Payment by Paypal.
http://www.forstle.com

Gator Games
Magic singles for sale.
http://www.gatorgames.com/

Grand Slam Sports Cards
Magic the Gathering cards and other ccg cards sold by the box.
http://www.grandslamsportscards.com

J.D. Trading Company
An internet storefront specializing in singles.
http://jdtradingcompany.com

KardKastle
US based dealer. Selling boxes, booster packs, sets, singles and sleeves. Also buys cards.
http://www.kardkastle.com/

Lazarus' Magic: The Gathering Auction
An automated auction site.
http://www.mtgauction.com

Magic
Offers Magic products for sale including boosters, starters, pre-constructed decks, sets, and singles ranging as far back as Arabian Nights. Texas, USA.
http://www.gamesnet.org/

Magic Card Sales
Individual selling a small selection of Classic Edition single cards.
http://www.angelfire.com/games/magicsales/index.html

Magic Card Singles
Features single cards and online virtual cards.
http://www.magiccardsingles.com

Magic Greats
Sells pre-made decks, repackaged random cards, and individual cards.
http://magicgreats.7p.com/

Magic Shack
An assortment of cards with shipping offered worldwide. Accepting all major credit cards as well as PayPal.
http://www.magic-shack.com/

Magic the Gathering Cards For Sale
Individual selling his 4th Edition to Prophecy cards.
http://www.angelfire.com/ky2/magicthegathering/

Monolith Lair
Individual selling a few rare cards. Site also has a trading board.
http://www.monolithlair.20m.com/

MTGFanatic.com
Offers purchasing as well as selling Magic the Gathing cards.
http://www.mtgfanatic.com/

MTGSale.com
Source for singles, boxes and accessories.
http://www.mtgsale.com

Omega Cards
Individuals selling cards from most sets.
http://www.omegacards.com/

OshkoshMagic.com
Cards and accessories including singles, common card lots, mystery packs, and dice.
http://www.oshkoshmagic.com

Smedlock.com
Offers a complete line of Magic the Gathering items for sale including singles, bulk lots, accessories and other related items.
http://www.smedlock.com

The Wagical Place
Specializes in singles for all in-print editions introduced after Fallen Empires.
http://www.wagicalplace.com/

TheEndGames.com
Sells MTG cards from alpha to invasion expansion sets. Includes a message board, deck lists, and a price list.
http://theendgames.com/

Wizards Cupboard
Older Magic the Gathering cards for sale.
http://www.wizardscupboard.com/