Hi guys ..
Answer by navinkumar is right, but I would like to tell avtar singh that follow the concepts of 'Materiality' and 'Professional Judgement' throughout an Audit.
Check only material Purchase / Sale transactions (keeping in view the time limit of any audit).
And apply your professional judgement i.e. any other suspicious matter if you come across, check it in detail. It may happen that after checking routine items, you may not get any thing but analysis of unusual transactions or matters may give you lot of stuff to audit.