I haven't used eBay for timeshare sales, but I a couple of rentals on eBay annually. I use a 10-day auction, timed so that it covers two weekends. That means it will end on either Sunday or Monday. I usually prefer Monday, because that gives me a better chance of being able to get some of those last minute question answered that might involve talking to someone at the resort. Sometimes the people who have the answers aren't available on weekends.
I usually end auctions about 7 pm Pacific Time. I figure that ensures that the maximum number of people will be home to bid at the last minute.