you are infringing on copy rights when you use someone else patterns then display or sell the quilt without asking for permission.
there is *built in* permission to make a quilt for your own use- or to give as a gift- if you want to sell it you need to ask-
if you want to enter it in a show you need to give credit where credit is due- and it is best to again-ask for permission-
some times you will come across a pattern that will say you can make up to ...5 quilts with this pattern to sell-
or some other- added permission- but on the norm- you should just contact and ask- it's not hard- and unless you are mass=producing the quilts most of the time the designer does not mind- as long as you remember to include the designer on the label.