I think it depends. There is never a one-size-fits-to-all answer in selecting your keywords. I think if you are running a shopping website plural key phrases would be the best strategy.
For example if you compare the searches of accessories and accessory here is the result
Google Trends: accessories, accessory
On my own understanding and experience, in most cases, the plural forms seem to be more searched than the singular, especially for e-commerce/marketing sites. However, this vary depends on the type of product you are marketing.