We have to identify External Entities, which I think are:-
Sales Person of Best Mart
Items/ Product Info
Another thing we missed are:
The Data Storage will contain following:
1. Item Master
2. Customer Master
3. User Master (this way I will create type which will tell me user types, such as
salesman, owners, administrator etc.
4. Vendor Master
5. Sales Data
Remember at the end of line it ask for the report generation, which is only
possible if you have sales data table where you will store the sales information.
Here are samples of context level of dfd. and 0 level dfd
Please make changes and made it as u understand it.