Product Bought is a function you can call to determine if the currently logged in customer has ever purchased a particular item. Product Bought Within offers the same functionality, but with the additional check that the purchase have been made within a particular number of days, weeks or months. Customers must be logged in for these functions to work.
Both functions are included in the download.
These functions may be used on any page in Zen Cart. Common uses would be checking for a past product purchase on the product info page or shopping cart page, and doing cross-selling there.
For more information, please visit this product's webpage.