I am new to domain trading. I have one question - Would I be sued (i.e is it legal?) if I buy some .app domains related to some popular apps of my country and list their google play store and apple store link with some ads on those domains?

I don't know if it's illegal, but unless you have a trademark on those names, you're almost certain to lose them.

Would I be sued (i.e is it legal?) if I buy some .app domains related to popular apps of my country and list their google play store and apple store link with some ads on those domains?

If your intention is to make some money and they have trademarks and already existing similar domains you are not acting legal! There were other people who already had the same thought as you... they failed!