Can I change the bootstrap default styles like buttons, alerts, tooltips etc. on bootstrap css file for themeforest item?
You can use everything that you are written your question.
How does ThemeForest work?
Try to study & research about themeforest hope you will getting good result
Actually, I’m not cleared. I asked about, can I change the bootstrap.css file for changing the default design of bootstrap components?
Of course you can. If this will not be allowed, all themes which are using Bootstrap will be the same (buttons, tabs, alerts, etc…).
I am recommending you not to change directly in bootstrap.css file - copy the style for button or any other element/component in your custom css file and there make changes which you need (override the default css)
Sure you can change if you can maintenance well why not.
Bear in mind that even if you edit default bootstrap, you still need to compile a premium quality design and executed site which takes a lot more than just restyling standard elements