How good is the battery in a used electric vehicle?

Batteries generally are holding up better than expected, but I'm interested in what happens when a problem shows up.

Are these dealers doing the test before they buy the cars? Do they have a formula for adjusting buying and selling price based on results? Is there any kind of guarantee associated with buying a tested car?

Is there a company that the dealers can take the failing cars to to get repaired? Does this make economic sense? Do they rip the packs apart to salvage the good modules for reuse?

Are the independent ratings accurate, and who rates the raters? Seems easy enough to compare them if there are multiple competitors, but is anyone doing it?

100K km on my Model 3, still over 90% of original rated capacity (and almost all of the decline - like through 94% vs 91% today - was during first 20K km, as expected).