Little is known about Sean and the Brandywines, indeed they only left this one Gary Usher produced 45 behind. According to 'Teenbeat Mayhem' they were from Tujunga, California.
'She Ain't No Good' is a choice cover version originally recorded by London mods The Knack. The latter group were signed to Decca records in England so it's not inconceivable that the song was given to their American associates for consideration.
Another London outfit The Clique also recorded 'She Ain't No Good' in 1965.
The other side (not sure which was the top side) is a fine folk rock rendition of 'Cod'ine'.
By the way, if you look at the picture of the 45 you'll see a price tag of 5 cents. I paid a lot more for it a couple of years ago!
Sean and the Brandywines - Cod'ine