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.
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 completly unfathomable.
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?
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...