On the registraion forms it will say what the endorsements are, the only time you need the letters is when actually putting them on a litter you are registering, and it explains it on the form.
If the seller can't prove that the purchaser was made aware of the endorsements, the KC is unlikely to enforce them. ie - if the purchaser requests that they are lifted, the KC probably will lift them.
This is correct as I have just had an endorsement overturned by the KC because the breeder did not inform me of the endorsement (the breeder needed a signed agreement) and the breeder did not have the pup in their physical possession at the time of registering the endorsement. You can get it put before a committee in the KC if you feel an endorsement is unfair.
I believe the KC will not always automatically lift the endorsement if a breeder cannot prove that they made the purchaser aware of the endorsement. I think they judge each case on it's own. I have heard of a couple recently on other groups where the KC did not lift the endorsement even without any written proof.