Classic

Atari 2600.com
Online supplier of classic video games, systems, and accessories including Atari, Colecovision, Intellivision, Odyssey, Vectrex, and others.
http://www.atari2600.com/

AtariAce.Com
Sells systems games for the Atari 2600 and 7800, original NES, and Genesis.
http://www.atariace.com/

Brooklyn Boys Classic Games
Buys, sells, and trades classic video games.
http://www.brooklynboysgames.com/

Good Deal Games
Sells games for many classic systems.
http://www.gooddealgames.com/

Ianoid Classic Video Games
Buy, sell, trade video games, especially Atari, Intellivision, Colecovision, and Commodore.
http://www.angelfire.com/id/ianoid/

Jadeite's Vintage Game Trading Post
Sells used Atari 2600, ColecoVision, and Commodore64 games.
http://rp_gamer.tripod.com/4Sale/

4JAYS Classic Video Games
Sells systems, parts, and games for Atari, Colecovision, Intellivision, Odyssey, Pong, and Commodore.
http://www.4jays.com/

Packrat Video Games
Provides vintage systems and games.
http://www.packratvg.com/

Player's Choice Classic Videogames
Hard to find and classic games shopping site.
http://www.playerschoicevideogames.com

Retro Action
Carries retro games as well as newer systems and software.
http://www.markhyde.plus.com

Retro Trader ltd.
Vintage toys, books, games, computers and consoles.
http://www.retro-trader.com

Retro-Games
Offers 16 and 32 bit consoles, handheld systems, and board games. Located in the UK.
http://www.retro-games.net/