It depends what you're reporting them for.
For example, if you believe that they're listing counterfeit items and report their listings, it's unlikely that eBay will take action because they can't see or authenticate the item in question, so have to err on the side of caution in case you're a competitor or disgruntled former buyer of a different item.
If you've purchased an item and return it under eBay's buyer protection then that won't go unnoticed, and you can leave appropriate feedback.