No, you cannot sell eggs in Pokemon HeartGold.
yes at supercheats.com/pokemon heartgold action replay codes
It's not possible, legendary Pokemon cant be bred.
no you can not have legendary Pokemon eggs unless you went to a Nintendo event
The PokeWalker allows you to walk with a select Pokemon from your copy of HeartGold and if you walk enough, you can capture extremely rare Pokemon (though not legendary).
there are many legendary Pokemon in Pokemon heartgold, but the best is HO-OH
I put a link to a guide all about legendary pokemon in HeartGold in the related links.
No, you cannot sell eggs in Pokemon HeartGold.
In HeartGold you can catch Kyogre.
Articuno
mew
I don't think you can refight any legendary Pokemon except the wandering dogs.
You can't breed legendary Pokemon eggs on Pokemon SoulSilver or on any Pokemon game.
migrate and trade
it is not possible as legendary Pokemon are not breedable
Kyogre is only available for Pokemon: Sapphire on the GBA. Ho-Oh is the legendary pokemon available to you in HeartGold.
It is absolutely impossible to get Legendary Pokemon eggs.