Based on my related experience I don’t think most people would care where the corpse comes from thus would not require a killmail as proof of anything, if they want to buy the corpse they do so as long as it is legit and not just an alt renamed to seem the same or a totally different one.
Not entirely sure about contracts but it is guaranteed you can see the names of the corpses if you open a direct trade window and put them there, at least the last time I sold corpses where the buyer wanted to check their names used this method and worked. The good thing is you can drag and drop corpses en masse so even if you have a container full (1,000) you can do it then the buyer can cherry pick the ones they want.
But it all depends on for what reason one buys them (see my related blog article) and never had anyone care about how the corpse was acquired even when they bought them as collector’s item.