You can definitely apply for the posts againts Commerce and probably economics too. Subjects like management n business administration are not taught at inter n bachlr lvl which are more relevant in ur context. Still, commerce is there and economics if they deem fit as per ur credit hours studied. Tips and books for preparation vary from subject to subject. Yours is a vastly studied subject which does not lack material in the market. N soon there will be piles of helping books available, but my personal experience suggests that they are of low quality. U should first of all gauge the idea of relevant subject matter. Define the areas of the subject that u are applying. For example if u apply for commerce Financial Accounting comes first, then Banking, Economics Commerce are very important. Focus on the relevant areas and dont forget its gonna be an mcq test which requires conceptual n to the point study. Make points n go through ur masters' subjects. Put ur effort optimally n avoid irrelevancies n a long term approach. N in the last, the best method to avoid negative marking is to avoid incorrect answers when u already have given correct answers of a considerable number of questions. Leave almost 5 to 10 of them unfilled that u may think wud not click by the way of chance. Try to construct ur lvl of preparation whr u can easily give correct answers of 80+ questions. n if u r pretty sure about ur answers u can easily leave the rest bcz 80+ is gonna be a very gud score for each subject n brings great chances of being shortlisted.