[dillondoyle]

I didn't! And it's interesting I just read the 1st player who declines to buy can participate in the auction.

How often would an opponent be willing to pay more than list price? I guess to secure like the 2nd blue property. Otherwise it seems like should always just decline to buy and hope for good auction outcome.

And if your opponent good to auction too. Though maybe mortgaging properties if you have a good opportunity is worth it.

What other rules make the game more interesting/complex that the 'young kid' version might not know?

[aidenn0]

You don't get any reward for landing on "free parking.". Rewards for landing on that spot are a common house rule that makes the game worse; injecting money into the game slows it down greatly.

[bombcar]

Often the “lesser” properties go to auction and it can be worth trying to get them for less than the mortgage value.

If you land on a high-value property you want to buy it outright mainly to deny the monopoly possibilities to others (high value being the orange, etc).