Okay, so, I work at my day job as a librarian, and spend a lot of time with copyright concerns. I've enclosed some links below about what I've discovered about quilts and quilt patterns and copyright. I'm also going to take a moment and mention that I'm not a lawyer, and this post is not legal advise. Please consult a lawyer before making any legal decisions relating to any information below or found by following the links.
The basics of it is: you can't distribute the pattern or the illustrations in the pattern without violating copyright law. However, a product made using a pattern is considered a derivative of the copyright work, and may not be covered by that copyright. So, theoretically, it's okay to sell quilts made using a copyright pattern, just as long as you don't include the instructions or the images from the instructions, and don't take credit for the pattern being of your own creation.
However, some copyrighted works may have additional restrictions as to the original intended purpose of the pattern. (And this is where quilters who sell things can run into a bit of trouble...) Many patterns will include the statement of for personal and nonprofit use only. This intends that the pattern not be produced in mass market environment. So, no having a factory in China making zillions of quilts for you using that pattern and then reselling them. And no claiming that the pattern was yours and yours alone.
So, generally speaking, like with writing a paper, give credit for the pieces of someone's work that you use, and please don't ruin someone's chances to make a living from something that they've worked really hard on.