THE HISTORY OF SUSSEX COUNTY, DELAWARE by Harold B. Hancock, 1976

p 59 is the only mention of Selbyville in this book:
"Another village located on a tributary of Indian River is Selbyville. A village
 began to appear when the Long family dammed Sandy Branch, a tributary of
 Sinepuxent Bay, for a mill pond to operate a grist mill and sawmill. The place
 was called Selbyville after Samson (sic; Sampson)) Selby who opened a store
 there in 1842. Asked by a Philadelphia merchant where to send goods for his
 store, he replied, "It will be shipped by Indian River vessel and then hauled
 to where I'm opening a country store, so just mark it Selbyville."

NOTES: 
3.1. The first sentence is inaccurate as Selbyville is not located on a tributary of Indian River. 
The previous paragraph described Frankford on Vines Creek which
 is a tributary of Indian River. 
Sandy Branch, where Selbyville is situated, flows
 into St. Martin's River by way of the Bishopville Prong.
3.2. The Longs were Isaiah <of Armwell of David, Frankford (after 1808)>, and his
 nephews, Zeno and Joseph of Benjamin of David. The year was 1817. 
(see the LONG and BUNTING folders for deeds proving this.)