My 2 cents -

• UPC codes are great for computers but poor for people.  Create alpha-numeric part numbers that mean something.  Put the item upc code in the 'mnfg part number' field.
• Items can be nested like accounts.  Create items for the 'type categories' and then crete sub-items for the actual active items you buy-sell.  Never buy-sell a 'header item'.
• Items link to accounts and accounts are used to generate most common financial reports.  That means the hierarchy you create for sorting and reporting item based info can be different from the hierarchy you use to report financial info.
• Do similar organization of your accounts - including creating header accounts for grouping similar types of entries.