Hello. I am a descendant of William Richardson's brother "King" John Richardson through his son Andrew Jackson Richardson. Have been searching for years trying to find some info about William and John in Virginia, with no luck I might add. Am also familiar with Sellers' book. I have found it to be a useful resource but is filled with many inaccuracies. I've learned to take any info in it with a grain of salt until independantly varified from other sources.
Sorry I can't provide any help on William and John prior to their coming to Marion Co., SC.