you bring up some really good points,
this one in particular 
"even though everyone knows a lot of it is out of the seller's control"
I notice that you have never listed an auction here on BYC, do you sell hatching eggs yourself? it's sounds like to have bought some eggs and not gotten all the "freebees" you wanted. when an auction lists 6 eggs, that is what you are bidding on, 6 eggs, and as you stated, everyone knows a lot of it is out of the seller's control. so how can you hold the seller responsible for the damage that happens once it leaves their hands. how does the seller know that you know how to hatch eggs, or that you aren't just saying they all got broken or didn't hatch just to get more free eggs. I personally only put up auctions this spring for 6 eggs because all but 1 rooster and 2 hens were left of my flock after a weasel hit us. it took several days for me to get 6 eggs that would be fresh enough to ship, if I sent 12, they would be over a week old. many people don't have a huge flock of a certain breed but have several smaller flocks of different breeds.   I am sure there is more to this story then you are saying.
I could tell you about the hatching eggs I bought that hatched as showgirls not silkies  and the replacements were more the same but I won't. I posted feedback on that person so everyone here will know and if you have a problem with a seller, they did not ship what you were buying or sent less then the auction stated, then you have gripe, but those extra eggs are just that, extras. sellers include them so you  hopefully will get the number of chicks from those eggs but unless the auction explicitly states will include extras, they are not obligated to do so and we can't control what happens to eggs after they leave our hands, whether by postal service you pick up and we also can't guarantee that all eggs are fertile even if our own hatches are 98%.