Because your eBay listings are exactly for 3 days or 7 days, whichever you select, the auction (or listing) will end on the same time you list it, unless you “schedule” it for another time (an option that is “Free” for casual eBay sellers after 1 May 2013).
It is always best to have your auction (or listing) end at a popular time with shoppers. Many shoppers will simply search “ending soonest” in order to refine their search based on the theory that the items that are just about to end will give them a very quick result of a cheap purchase.
Most eBay customers shop between 7 and 10pm all nights, and the most popular days are Saturday and Sunday. Monday and Thursday evenings also fair very well. Obviously you can try different times and see which ones sell better for you, but if you are an international seller it may be worth selling everywhere to capture buyers around the world until you know which days are best.
How long should you list your products for sale
One day listings are only worthwhile for very popular items that people are searching and bidding on all the time (like an iPad) or as a special event (like tickets).
3, 5, 7, 10 days are not very common with eBook resellers on eBay. They all appear to take advantage of the longest listing possible for eBay stores, which is “30 days” or “Goods Till Cancelled”. If you select “Goods Till Cancelled” you still pay every 30 days for a new listing.