You'd have to carefully read the designer's copyright wording.
For example, Bonne Marie Burns of Chic Knits is VERY CLEAR that you cannot knit her items and sell them anywhere, anytime.
But, honestly, if you found a hat pattern, and if you were going to sell it at a craft fair, why not just change one thing?
I think where people get into hot water is when they try to sell things over the internet. Person-to-person at a craft fair doesn't seem very dangerous.
However, I'd personally try to change something even for selling at a craft fair.
We all know that it is impossible to make money at selling hand knits, that's why designers are so protective of their designs. It is the only way they make a living on knitting, and a hard one at that! I've 'designed' a few things, and believe me...I'd cringe at the thought of converting it to print for someone to purchase and follow my train of thought!
It is a hard thing to do, IMO! (writing out a pattern design that makes sense!)