Wow Eric, not quite the response I expected from you. But I think you're spot on.
Unless someone personally goes into the locker room and removes the jersey from a player's back after a game, due diligence is always required.
Team sold or player sold is not a justification for failing to do your homework nor is it a guarantee of authenticity.
If the Jays sent me a 1990 Russell jersey, for example, the last thing I would think is that it's legit. My first thoughts would be that it was made for a photo shoot, promo, charity giveaway and I'd call them to up to discuss why it isn't a Wilson. To blindly take a jersey from a team or player and say "well it shows use and it's from the team so it must be good" is naive at best and dangerous at worst. To then go a step further and issue an LOA for it based solely on those 2 aspects is irresponsible. Fonts, styles, patches, sizes, rosters, etcetcetc all need to be checked regardless of provenance.
Your quote re: Gionfriddo was amusing. Not only do some teams, managers, players simply not know what they're really dealing with, but some intentionally mislead for the sake of profit. What about those jerseys directly from Manny Ramirez? Or the jerseys and bats signed by Arod as "game used"? Players will order spares to give away to friends or to sell privately, teams will order spares for photo shoots and charities. These spares find their way into hobby. They're hardly gameworn (or even game-issued) even if they are directly from the team/player. The source of an item is only 1 aspect in determining it's authenticity and it's naive to think it's the only aspect you need to consider.

Rudy.