You know the person and can trust that you will get the product. Use other Atari enthusiasts. Atari fans hold conventions all across the country. Try the Atari Events page for listings. Prices vary greatly depending on what source or region of the country you are in.
Prices seem a bit high at flea markets and on some retail sites on the internet.
Prices are lower in garage sales and local video game stores. 7800 items are rarer and so are probably more expensive. The Jaguar CD unit is more expensive because it is rare. Two factors affect price, supply and demand. The most popular games include Alien v. Predator, Tempest 2000, Rayman, and Defender 2000.