*In my experience Monopoly games played by the official rules... Any thoughts on what we were doing wrong?"
I'm guessing you weren't actually following the rules[1]. If you do follow the rules you'll find liquidity gets sucked out of the game by continual reselling because you are only allowed to sell unimproved properties, and have to sell houses & hotels back to the bank at half price.
That rule was followed. Given that we were playing with 3-4 experienced players, monopolies were rare (everyone bought essentially every available property landed on that was still potentially monopolized) and most targeted the sweet spot of three houses. Monopolied properties weren't traded often, but even then that's not a lot of liquidity to sacrifice with ~$800/board cycle going into the game via passing Go.
Once a few monopolies did crop up, it was the remaining unimproved properties that would be swapped most often for ludicrous amounts of money to allow, say, a cash-poor player to build a few houses in the path of the current leader.
I'm guessing you weren't actually following the rules[1]. If you do follow the rules you'll find liquidity gets sucked out of the game by continual reselling because you are only allowed to sell unimproved properties, and have to sell houses & hotels back to the bank at half price.
[1] http://richard_wilding.tripod.com/monorules.htm#sellingprope...