OK, I must admit, I did no research to see how often any of these specifically came up, they're just what I've observed in the time I've been buying on eBay (and selling as well).
1. No Photo - Or a Really Bad Photo
This mistake is really inexcusable... anyone who's selling on eBay should have a digital camera of some kind -- even if it's just the one on their cell phone. Having a photograph (that's not "borrowed" from somewhere else, see below) of the actual item for sale is very important... and just as important is making sure that it's a decent photo! I don't know about you, but I'm not prepared to spend much on an item where the seller couldn't be bothered to take a photo that wasn't all blurred out!
2. Photos That Aren't of the Item Itself!
This one really bugs the hell out of me, and it's most often seen with electronics items, whether it's computer stuff or cell phones. It's obvious that the photo's been saved from the manufacturer's website, especially for sellers that are selling a lot of the same thing, and the exact same photo is being used on all of them!
3. Poor Title of Auction
Let's say that you've decided to collect View-Masters (as I've started doing this year), and you see an auction listing of "Vintage View-Master and Reels" -- what does that even tell you? At the very least, the number of reels included in that listing should be there, i.e., "Vintage View-Master and 20 Reels". By listing the number of reels, I don't have to waste time looking at the auction description to see just how many reels are being included with this item. What would've been worse would be if this sample auction had just said "View-Masters" and nothing else!
4. Misleading Auction Title
Yes, there are misleading titles... I don't know how often it's misleading on purpose, or if it's accidental. Is something that was manufactured five years ago or less really considered "vintage"? For that matter, instead of using "vintage," it would be smarter to just list the year -- that's only 4 digits, saving you 3 digits for something else on the title. Abuse of words such as "amazing" or "incredible" are misleading as well, in my opinion... if for no other reason that I've never come across an auction using words like that where I felt the words were used appropriately.
5. Inaccurate Auction Information
Let's face it, you're going to get better results on an auction if you have the correct information. I wish I had a dollar or every time I've seen "Marvel" included in the description for a Batman item, or "Disney" used with Casper or even the Looney Tunes! If you can't list the item correctly, you're banking on someone just coming across your listing at random... and it's not going to get you bids. Another inaccuracy I see is just plain listing an item by the wrong name... I've seen Give-A-Show Projectors listed as "Movie Projectors" -- BZZT! Wrong answer! There's no "movie" about it! A little bit of research would probably not only get the correct name for something, but you might end up learning more about it that could be useful in the auction description.
6. Incomplete Auction Information
Just as bad as being inaccurate is being incomplete. If I see an auction listing for a group of stuff and I see the phrase "too much to list," I close that window right away, because if the seller can't be bothered to provide this information, I can't be bothered to give them my money. Everything that's being offered in the auction should be listed, with the description included (and spelled out as much as possible).
7. Not Using Delivery Confirmation (or Other Tracking) With Shipping
This is one of the dumbest things I see sellers do. As a seller, you are only protected by PayPal if you have some kind of tracking included with your shipping. How else are you going to prove that the item was delivered as promised? This can be free if you know what you're doing (see below), maybe $0.19 or so. It's more than worth whatever it costs -- in the past, I've had buyers claim that they hadn't received an item, and when I pointed out that the USPS said it was delivered, suddenly, that buyer would happen to find where the package is!
8. Not Buying Postage Through PayPal
Along with #7 above, this one is something I just plain don't understand... eBay requires that you offer PayPal as a payment method (which makes sense, because they own PayPal, and it does speed up payments a lot). What a lot of sellers don't seem to get is that when you pay for the postage through PayPal, the postage costs less than if you bring a package in to the post office and pay for the postage there. Not only that, but on some forms of shipping, Delivery Confirmation is free (and for the others, it's cheaper... a LOT cheaper)! Even better, when you buy postage through PayPal, you get an address label generated with the postage on it and the DC that you can print, cut out, and tape on (or, if you use label paper, stick on)! And as if that doesn't seem like a good enough incentive, the auction will automatically be updated as shipped, and the Delivery Confirmation number appears on your "my eBay" page (as well as the buyer's page) as a clickable link to track its progress. If this isn't enough incentive to use this, you also have the advantage that you don't have to wait in line at the post office, you can usually just bring your package (or packages) right up to the counter and drop them off, since the postage is already paid!
9. Overcharging for Postage
As noted in this blog a few months back, some sellers on eBay ridiculously overcharge for shipping, hoping to make extra money on their sale without eBay getting a cut of it... completely ludicrous! Now, I can handle a reasonable handling charge added onto the postage rate... even up to $1.50, if you have to purchase supplies for postage such as packaging tape, padded envelopes, bubble wrap, etc. (personally, I keep my handling charge to a dollar or less, usually rounding to the next 50 cents). I have purchased items from sellers who charged me $5.00 for shipping, and when I got the package, the actual shipping cost turned out to be $2.25 or so, and the item was in a plain envelope. That's ridiculous marking up (another argument for buying postage through PayPal is that you have the option of not having the postage value on the address/postage label, by the way). Of course, if the shipping price on an auction is way too high, the auction should be reported.
10. Outright Lying
Most of this tends to happen after an auction has ended. Ever bought something on eBay, and waited and waited for your package to ship and arrive at your door, and when it does arrive, you check the day it was shipped compared to when the auction listing said it was supposed to ship, and found it was shipped much later than promised? Or found that you were supposed to have something sent Priority Mail, but it was sent First Class, Media Mail, or Parcel Post? That's usually indicative of outright lying, and the only reason some sellers can get away with it is that buyers don't always keep track of things the way they probably should.
Now, I'd imagine this is not a complete list of the bad things some eBay sellers do... and there may be things that you've personally experienced that you think is at least as bad, if not worst, than the 10 things I've listed here... if that's the case, please feel free to share in the comments!