Ankit i would advice you to try other accounting softwares too, Tally is not solution to everything,, specially when things are not in definite format,,
Also if you want to go for Tally, you have to make some compromises,
Like if you or your guy is giving his full time at that shop you may enter Sale of each product ( In this case even you won't have bill, you will be 96% correct.
Also if the Turnover is below 1 Crore, Audit and AS will not be applicable for such shop making it easy for you to work.
If you go there for part time or once a day something like that, you may ask shopkeeper to keep a record of items he sale during that time (Qty.*Rate) or if Rate is same for for all then Qty.
See you have to improvise your working strategies according to the situations.
Work out the solution by asking the owner and work for his needs...