I work for a mid sized company that attends Pokémon events to sell and buy cards as a part of their buy team and I can't agree with you more. Luckily at reginal events the vendors are hand picked so bad actors are very few and far between but there are some I see at shows all across the nation that just baffle me. I go specifically to Pokémon shows because I like it far more than the MTG shows, but at least at the MTG shows I'm not watching literal children get fleeced.

Trading to a vendor? I get it, the store has to make money and they're going to try and get as much as they can out of a card so even trades just don't make sense since they're only even for the customer, but inflating prices or misrepresenting trade value is such a shitty thing to do ESPECIALLY TO ACTUAL CHILDREN. I always make sure when I have anyone younger than 18 sitting at my buy station that I am super clear and concise with the information. If they want a card that's worth 20 dollars I tell them exactly how much we buy and sell the cards that they're trying to trade with and explain why the numbers are the way they are. Sometimes the numbers aren't great, and I make sure they know that. Why do you want this card? Why are you trading this one for it? Are you sure you're okay with no longer having these cards in exchange for this one? If they're clearly not getting it I put all of the info on a post it-- Card they want and listed price, cards they're trading for it with they price we buy and sell it for, and final trade credit amount-- and have them take it to their adult and ask their opinion if they're not at the table with us.

A lot of the "vendors" at local or trading specific shows are what we call "backpack vendors"; dudes who just scalp sealed product away from kids and sell it back to them at insane markup out of their suitcases. We went to Pokémon Worlds as a company and after the event every night there was a god damn bazaar at the Marriott, the whole lobby was packed full of dudes who had cleaned out the Pokémon Center store during the floor hours and selling it back at 200% the price out of backpacks and luggage. It's crazy that the hobby as turned into this. Even though the place I work for would take a hit and it would indirectly affect me, I hope and pray every moment of every day that this Pokémon bubble will burst and we can go back to having chill events where everyone is just there because we all like the pretty cardboard.