Silhouette have recently updated their Commercial Policy with regards to you using Library and Download Designs in projects that you sell. The basic policy is that if you use the designs in cards etc that you give away then you are ok using the downloads without needing to buy a commercial licence.
You can draw inspiration from anywhere and anything you design from scratch you can use in projects you sell. There are also thousands of free designs without restrictions available through a variety of sources.
If you'd like to learn more you can via this link.