Did they copy phrases, tags, titles, paragraphs? Most of the description adding their own text in between etc? I suppose it depends how much and what they are copying...
If you paid a copywriter to write your listing descriptions, don't they hold the copyright of their text? I don't know how that works, just wondering...I would ask them first.
Copying listing descriptions is not allowed. Depending on the circumstances, that can be considered plagiarism or copyright infringement. It also creates duplicate text on Google and that will hurt you, or both of you, in searches.
You can put a notice about copyright in your policies or in your listing descriptions, you are allowed to. But unfortunately, copy cats have no shame.