Can I sell jumpers I have knitted from a knitting pattern in a shop?

Probably not without obtaining the permission of the copyright or design right owner first. Whilst the point of publishing a pattern in a magazine or selling a pattern in a shop is clearly for people to knit the garment, it is usually only intended for a few garments to be made for e.g. family members, and not for multiple use for commercial gain.

There may be copyright or design right in the garment or copyright in the decoration on the garment that would entitle the rights owner to control what happens to the garments if they are knitted to be sold commercially. However, check the small print on the pattern as this might help you decide what you can do.