I found this comment to be useful, so here it is:
I find that if something is unusual, it is best to use the 7 day auction format, so it gives more buyers time to find the item.
If I’m selling something more common, I prefer 3 day listings. A lot of listings don’t get any bids until the last few hours (or minutes) of the auction, so I don’t see a reason to spend 4-6 days waiting for my item to make it to the first page of the search.
My main reason for choosing something other than a 7-day listing is because I want my listing to end on a certain day. If Thursday was my big selling day, I could list 7-day auctions on Thursday, 5-day auctions on Saturday, and 3-day auctions on Monday and all of them would end on Thursday.
Also, if someone else is selling something similar to what I have to sell and they are charging a lot more than I plan to charge, I wait and use the 3-day listing format so that I can place my item to end just a few minutes after their item is scheduled to end. Thus, my $50 item is listed directly below someone asking $75 for the same item.