one other you have to be very careful of now that was not a big issue back in the 80-90's when crafting was huge is "copyright infringement"....
you can not take a Lazy Girl wonder wallet pattern and mass produce them for sale at shows or on websites....that is a HUGE no-no...
you also can not use "licensed" fabrics LIke disney, cartoons, colleges, etc to make mass produced items ...and not just the fabric but anything...like a machine embroidered bib with mickey is out, or wooden wagon with OSU on the side....all of those things, characters, schools etc, are licensed and you have to have a lot of money to buy into their programs that allow to sell items using their colors/names/characters....
Etsy, Artfire and Ebay are hot beds for copyright infringements and schools and designers are always on the lookout for violations!
Having said, there are some designers in the modern world that recognize the fact that if you sell something made from their pattern, then others might want that pattern as well! Kaye Wood is one and Nancy Halvorsen is my favorite! Nancy is from Art to heart fame and she gives you permission to make 12 of each item in her books to sell. Kaye will grant you permission in writing to produce "X" number of the item. IF you find something you like and you think it will sell, take a moment to ask the designer if it is ok! If they say NO, then you really need to find something else to make! But many are really good about it.