You’re right actually, I see now. This is because everyone’s coin button and buy button are the same button. You need to reference different buttons for different players, as I already commented in my code snippet above.
-- This sets up a connection on the same buyPart as everyone else. Somehow you need to
-- reference an unused buyPart unique for this player. Without seeing the setup of your game,
-- I can't tell you how to do that.
...
-- Again this is the same collect cash part for everyone, typically tycoon games have each player
-- have their own collection part.
My script doesn’t fix the issue, but it does have comments for where the problems are and an example fix. But it’s incomplete without knowing the full structure of your game.
The tutorial I linked does indeed account for multiple players correctly because it tracks ownership of the plots.
I’m sure it’s all confusing at first, but the tutorial really holds your hand the whole way and you should come out the other side with a much better understanding!