Re: Purchasing, selling and Stocking of products.
Depends on the entire biz requirement, too big a thing to discuss all at once in a forum like this.
There's really just one way to answer your question. Make your best guess, build the entire model taking a systematic approach like that described in
http://www.artfulsoftware.com/dbdesignbasics.html. When the model looks complete, build ("forward engineer") the database, populate it with sample data. Then write and test every add/edit/delete/select query you can anticipate; doing that will expose problems that need correcting. When you can find no more problems, it's time to bring in the alpha testers.