Free to use
You are free to sell your own designs without any limitations unless they infringe on existing copyrights and trademarks. If you use your own designs and market them online ensure you watermark your pictures and protect your intellectual property from potential theft/abuse.
If you purchase a design from Etsy or other designers the usage is probably for personal use only. If you intend to resell anything using a purchased license you will need to ensure that you are able to do so. Check with the designer and be sure you have permission or obtain a commercial license (you can keep on file) so that you can use a design as intended. Designers should be rewarded for their work. This requirement is the same for designs, fonts and/or sayings, taglines, etc. Using NFL, NBA, Disney or any other large brand is forbidden unless you have license agreements in place.
Using NFL, NBA, Disney or any other large brand is forbidden unless you have commercial licences in place. These are expensive, restricted and protected! It is not if you will get caught selling items with trademarked designs but when. Fines can be steep and equipment can be confiscated. Be aware, be warned!
Cricut sells cartridges and both Silhouette and Cricut sell electronic designs through their stores. These have, at least for Cricut special licensing again for personal usage only. For more information on Cricut’s Angel Policy you can read here us.cricut.com/home/legal/angel-policy