Flanker lines up on line of scrimmage, just prior to snap flanker goes in motion backward on an end around. Illegal?Also, Can only players off line go in motion before snap?

If the flanker goes in motion he must be at least 5 yards behind the line of scrimmage at the snap if he started from any position not clearly behind the line...NFHS 7-2-7

This is a commonly missed rules infraction.

Or he must shift and come set again to stablish himself as a back before he goes in motion.

Also, the linesman and line judge will need to be sure that there are 7 players on the line of scrimmage if the flanker goes in motion behind the line.