Tell your uncle the wood stays on the board. Once you start bartering for chores, someone's gonna try to trade a sheep for a kidney next. He's taking the "trading phase" way too literally.
Nah your uncle's actually onto something here, opening up the trading makes the game way more fun imo. Strict board-only rules kill the creative deals that make these games memorable in the first place. If everyone agrees to the terms, trading a sheep for taking out the trash just shows good strategy. Games are supposed to adapt to the people playing, not the other way around.
Actually the fun creative deals are already in the rules, like future promises or card swaps! Going totally off-board with chores or favors can pressure people who don't want to play that way. The game balance gets weird if someone trades a sheep for a week of dishes, because now their in-game win affects real life. Keeping it to the game resources lets everyone agree on the same playing field from the start.