If you want to sell card, the most important thing is to look at the copyright/angel policies and commercial use guidelines of the individual manufacturers. There are some that are not allowing or are severely limiting the use for cards for sale. With some manufacturers you need to apply for a permission. Sometimes the use can also be conditional (e.g. you need to add a certain wording to the back of your card).
As a general rule you should be extremely careful and if in doubt stay away from any dies or embossing folders with licenced characters (e.g. Disney, any characters from TV or films, etc.). Most of the time these are heavily copyrighted and taboo for cards for sale.
This research will also be necessary for other craft items your are using. And unfortunately there is no general guideline. It varies massively from manufacturer to manufacturer and sometimes even from range to range (e.g. with Docrafts or Crafter’s Companion).
Apart from that it is best to go for a system that can take a variety of different types of dies. I would, for example, not choose a machine with a width of less than 6”, as they limit what you can do with them .
