Most sellers on eBay dont do auctions anymore and actually a 7 day auction could only end on Shabbos if you started it on Shabbos as its exactly 7 days later when it was listed.
What is disturbing here is people are comparing B & H to other sellers and trying to impose a CHUMRA on others when they clearly dont know the Halacha.
Also there is something FBA which is Fulfillment by Amazon where you ship the stuff to Amazon, Amazon sells the stuff, collectes the money and Ship the item and give you the cash. You dont do anything, There is no VACATION setting for this service, the selling is up to Amazon.