How do I forecast sales data down to the individual item?
Do you want to forecast sales by day over a series of days? That would be more like ARIMA. Do you want to forecast "How many widgets will we sell in the next month"? That would be more like a regression problem.

As an aside, if the retailer you're working with has a large assortment of products that changes over time (e.g. seasonal clothing) then you should also look into forecasting by product category and not an individual product. It will be much more accurate.