If her listing photos and description are identical to you it breaches ebay policy and you could contact ebay trust and safety to lodge a complaint. 

 If she has copied your invitation designs but listed it differently to you, it would be a copyright issue. You would need to take this up with a lawyer as copyright is not directly an ebay issue, although they will remove items if a breach of copyright is proved.

Sellers are allowed to have seperate ID,s to sell identical items. Many sellers on the boards ( myself included ) have more than one selling ID. 

Good luck with this one. It would be very upsetting to find someone had stolen your own designs and was selling them in competition with you.