There is a market, but there is also a limit to the number of Pokémon a single person can train. And of course, certain species are more popular than others, so certain eggs may take longer to sell.

[Cecil does react to Mr. Beans' grand entrance... with a smile and a wave. What a sweet, clumsy little hellhound!]

You absolutely need a male and a female of the same group if you wish to obtain an egg, with one exception: Ditto. A Ditto can breed with nearly any species, even ones that have no gender... but a Ditto's offspring will never know extra moves upon hatching, which will lower their value. Not that it stopped Gogo from dallying with a few of my Pokémon, this time.