What is the supposed threat model here?

Mr. Rich Guy sells me his personal device he used in the previous year because he wants new shiny phone, but he may have the very slightest chance of being a super evil genius? The government selling tampered phones on ebay, when they could just.. go directly to vendors and put their backdoors directly into new phones/software?

Sorry for the light snark, but this attack vector seems way too complicated for not much benefit. Unless you are some very VIP person being personally targeted.