Why would you ever reject buying a Star if you had the coins to pay for it? If you're below first place and buying a Star will give you the lead (either on Star or coin differential), that makes you the prime target to have a Star stolen if someone else gets Boo's attention. If it's late enough in the game, losing said Star to Boo could be the winning difference.
One example in Mario Party 7 where the optimal move is to not buy a Star can be seen here. Yoshi, who had zero Stars at the beginning of the turn, passed a Star space and landed on a Pink Boo space, which results in you losing a Star (or 10 coins if you don't have one). Had Yoshi bought the Star, he would have wasted 20 coins and given an opponent a free Star. Since he didn't buy the Star, he only lost 10 coins, so he was actually better off both coin-wise and Star-wise!
And in Mario Party 5, notice that DK has his own space and this is the only game where he sometimes stops Bowser when someone lands on a Bowser space with a punch. And in said game, it takes place in the land of dreams and wishes, so DK's wish is to help people.
Which explains why DK's favoured item is the Bowser Bomb...
It seems a bit odd that in Mario Party 3, the Mushroom Jeanie can easily move the thousand-year-old Superstar status-granting Millennium Star like it's nothing. Until you learn that the Millennium Star that sells Stars on the board isn't the real one.
Why is Bowser so much more amiable in Super Mario Party than in the other games? Well in most of those other games Bowser's beef was the fact that he kept getting left out when the invites were sent to the parties. Now that he's actually been invited he's a lot more mellow.
Mario Party 8's "King of the Thrill" involves four players on several platforms over a very deep abyss. If one member from both teams are knocked off, the remaining players pummel each other, and at the end, you can see that Toadies have rescued the losing team, but not the winning person's teammate.
Also, in Mario Party 7, the game Ghost in the Hall has a bit of Fridge Horror. What happens to the characters who 'lose'? To explain, the object of the game is to run through a dark maze in a haunted house to get to the exit, with a lantern (and a few path-blocking Pink Boos) to help you. The winner comes outside. The thing about the game is that, due to the fact it only shows one character outside the house, going inside, this presumably means the other characters are in their own houses. So the losers...well, don't think about it too much...
Boo, unlike the others, is levitating. It should be impossible for it to lose mini-games that involve tasks like, say, avoiding being hit in the face by a rotating pole.
Maybe it's playing fair? Mario seems to keep his jumping on-par with everyone else here, so it's not completely unfathomable.
On a related note, in Boo's Horror Castle, the animation that plays when a Boo catches up to the players (he opens his mouth, sticks out his tongue, and the mini-stars fly up towards him) suggests that they're eating the mini-stars. How can they do this if, as ghosts, they have no need to eat?
Need, no. Want/Desire to? Yes.
In 5, on the last five turns on Bowser's Nightmare, Eldstar complains about Bowser interfering by implementing the last five turn effects. While he might have room complain about that on the other boards, with this, it's HIS BOARD, how is him implementing an event, that's a staple of the series, something he has room to complain about?
Eldstar doesn't like the fact that Bowser is trolling the players, since Eldstar is the host of the party altogether.
Shy Guy and Kamek are stated to be working, and playing for, Bowser in 9's Story Mode. So why are they beating up the minions Bowser sends after the heroes and then fighting Bowser Jr. and Bowser himself?
In the Chain Chomp boss fight in Mario Party 9, the characters aren't strapped or chained or whatever to their carts. So why don't the people who are facing the Chain Chomp simply bail out of the cart?