Lower, Mark A (1860) Patronymica Britannica: a dictionary of the family names of the United Kingdom. London: J.R. Smith. Public Domain.
SELLENS
As described in historical dictionaries
Patronymica Britannica (1860)
SELLEN", SELLENS. I can prove by tlie evidence of parish registers, &c., in Sussex, that this is a corruption of the ancient surname of Selwyn.