Are retailers that sell new and used products allowed to open a product before completing the transaction?
The retailer in question sells both new and used products. It differentiates between the two by virtue of the product being opened. If the product is not opened, it is new. If it is opened, it is used. The retailer charges different amounts for a used product than it does a new product.
By the retailer opening it before you take possession of it, is this not in effect selling a used item at new item prices? Does this violate any laws or are there any Customer Protection laws that prevent this behavior?
The retailer can do anything they want to a product before they sell it including inspecting the product to ensure everything is included in the package.
A product is used if it has performed it’s intended function for a customer or has been used as a demonstration product. It is not used simply because the package has been opened.
To answer your question, no. By the nature of your question there are two thoughts that come to mind. One is that you are too cheap to buy something new. The other is that you plan on removing parts and taking the item back and claiming they were not there. If they open the package for inspection and all parts are there, this limits your scheme. Great policy that store has.