The aliens on this map spawn regardless of what you do. The protoraptor eggs are able to be bought/sold at that price to allow the player to 'bank' their cash, if a lot of colonists are selected and cash is being drained through colonist's wages.

Does not selling the eggs stop the protoraptors from showing up?

Yes, if I remember correctly, this map does have never-ending alien hordes.

is this a tower defense like game with all types of aliens coming to base attacking?