Sellers should be sensitive to the copyright of the pattern that they are using to complete an item and then in turn sell. Some patterns have very explicit information about selling for profit. Many just require credit given to the design creator while others prohibit sale for profit. There is one particular pattern called Magic Tiles that does not allow sale for profit and the designer has taken it upon herself to scout out use and go after those that are using it for profit.
This is a very touchy issue and if you are using someone else's designs for selling be sure of the stipulations/copyright clauses that they request.
On the other hand there are many of the older patterns that do not have a copyright. Many times a designer has taken an older bock and reworked it into a 'newer' design, whether it be in color or placement. These older patterns are considered to be already part of the public domain and copyright does not apply.