A major part of an eBay listing is a picture of the item we're selling.
Like everything else, I have my standards, which, also like everything else, are better than some & worse than others. We sell mostly old postcards and antique photos, and getting a good quality picture of them to go in our listings is not always easy. I feel that the picture is vitally important though, maybe the most important part of the listing so I try to do a good job.
So here are my rules for our listings. The picture should be in focus, with no glare from lights etc., for postcards & photos, both sides should be pictured even if there is no written information on the back side, backgrounds should be minimized, the colors as displayed on MY computer monitor should accurately reflect the colors on the actual item.
In Focus: this seems like a no brainer, but I buy things off of eBay etc, and I am surprised at out many out of focus pictures there are about, and also how many pictures have a glare from a flash. This lack of attention to detail does not reflect well on the seller.
Both Sides Pictured: I take pictures of the front as well as the back, making sure in both cases that all corners and edges are visible. Lots of times it's the corners & edges that determine if an item is in good, very good or excellent condition.
Minimize backgrounds: When I take pictures of my cards & photos, I crop them to the point that you can see the whole thing (including edges) and not much else. The less background the better. Personally I like back grounds - I like looking at the little details of someones life that have inadvertently slipped into a listing, but backgrounds are a distraction. So keep it simple, and as minimalist as possible.
Item colors: Getting the colors & general look of an item exactly right is unbelievably hard. The best I can generally hope for is to come as close as possible, and make sure I'm not way off base. I'm not a professional photographer. I use a simple light box, and an old 5 MP digital camera bought long before we started in eBay. I've also become good friends with a simple freeware photo editing tool. Frequently, what I see in front of me, and what I see on my monitor are two different things. I edit the photo to try to get what's on my monitor to reflect what I see in front of me. But even if I'm successful in doing that, what other people see on their monitors may not be exactly what I see on mine. So I do the best I can, and so far it's been ok. 19th century Cabinet Photos and CDVs can be especially tricky, because even though they fall under the broad term "black and white", they aren't really. The ones that have survived in good condition have lots of subtle shades of light in them - some of those photographers were very good. My little camera cannot reproduce all those shades accurately, so I try to coax it out with the photo editor. And I have to be very careful to not misrepresent the picture in the process.
Generally if I'm thinking of buying something online and the picture is not good - out of focus, glares, doesn't show good detail etc - I move on. I figure if I do that, lots of others do too, so I try to take a decent picture. A picture is the only opportunity a potential buyer has to see an item they are buying online, and as as seller I may be competing with others selling the exact same item. The better the picture, the better my chances are.