If he has just your acc. number nothing he can do , don't worry. If you give someone Cheque also they cannot do anything to your acc. (apart from deposit money!)

They will probably try to send you some kind of fake notification pretending to be from Maybank that money has been deposited, and think you're stupid enough to send the item without actually checking funds have arrived in your account.

Just ignore the seller

Mudah is viewable outside of Malaysia, no problem to access it from UK?

Never trust Nigerians or other african countries when doing any business / selling .. always something dodgy.

There is, check out http://www.419eater.com/

Spend a few hours reading