Here are some of my experience in using ebay to sell stuff.
Ebay offers 1, 3, 5, 7 days auction listing. Which option to use depends on what kind of items you are selling, and how often do you sell the same thing. Listing for too long sometimes is just a pure waste of time, because I found people always only start biding the last day, even the last hour. Listing for longer can probably attract some more attention, but if the buyer wants it urgently, they will buy it from somewhere else. Also it gives the buyer a chance to think it through, they might give up the intention of buying or biding on it.
Unless it’s a rare item, it’s very hard to find the same thing elsewhere, you will probably gain some watchers. Or if the item is not very popular, you want to list it longer to gain more exposure.
For example if you are selling an iphone, it’s very popular, use 1 day auction is best. You save a lot of time, and there will be a lot of people biding on it. However, if it’s a ancient gold coin, you probably want to list it longer, so you gain some more watchers who might bid on it.
Buy it now option
With this option, buyer can buy your item right away, with a cost that is at least 40% more than the initial biding price. But you need to pay an additional fee to enable this option in eBay.
I usually have this option on when I am selling for long duration, and valueable items. There are always desperate people looking form items, and it’s a great way to “gamble” for a much higher profit for the cost of only 50p.