1. They don't do what they say they're going to do. This can apply in several different ways. First being, they don't ship or send out invoices in time. They take multiple days to send out an invoice or also take multiple days after payment is received to ship books. If there's an auction, they don't start when they stated they were going to start or on the flip side, don't close or end the auction at the time they said. Think about making start and end times that are convenient for your customers. So starting/ending your sale during business hours regardless of what time zone you are in. Just a small thing to be cognizant of when you're planning your sale. Plus take in account the day of the week as well. Experiment and see what will work best for you and your customers.
Also on the dark side of selling, I've seen sellers charge priority pricing for shipping and go the cheap route of using media mail. Just plain shady. There is no quicker way to kill your reputation as a seller other than a horrible packing job. I've also have heard of sellers upcharging customers for PayPal fees. In my opinion, as a seller, you should take that in to account of pricing your books. Also some sellers tend to also not bag/board books or pack what they've listed in the picture. For instance I had a seller in the past show books that were bag and boarded. However when they arrived I got several books stuffed into one bag/board.
Set realistic time frames you know you can honor no matter what. In terms of start/finish and also when your books will ship as well. I recommend making sure you separate books from your own collection and others to avoid confusion. If you state you're going to ship one method, make sure you stick to that for everything. Also if you miscalculate, it's ok to be human and mess up! Just message your buyer/customer and make sure there's clear consistent communication about everything. Everyone makes mistakes and everyone appreciates honesty when that does happen.
2. They don't research how to pack and ship properly and also create a system for it. I'm a firm believer in the golden rule for shipping. That being I ship books as I'd like them to be shipped to me as if they would be my own as they once were. Plus you can learn everything on YouTube. Take the time to do the reasearch. It's worth it to spend 20-30 minutes watching videos versus possible profit loss from having to refund buyer's money or accepting returns of damaged books. At that point everyone loses. So take the time to do it right and pack for worst case scenario.
I learned how to pack after buying hundreds of books online. It gave me firsthand experience in what did and what didn't work. I highly recommend Gemini comic mailers for up to 10 books. Anything over that, I reinforce medium sized Priority boxes. If you want more advice, check out my eBook.
3. They either overprice or underprice their books. If you go one route and go high, you can risk no sales at all. And you can possibly push away lots of buyers from returning to your sales platform because they'll think all your books will be overpriced. There's nothing wrong with shooting high though. As beauty is in the eye of the beholder and some times demand is high for a certain issue. Now if under price your sale you risk losing money. There was a time when I wanted to attract buyers as I lost my confidence in selling after a few sales where I didn't sell anything or turn a profit. So I set a number of books at only a dollar hoping they would go for much more than my buying price. Only to find that I lost my initial investment in the books.
Research your books with a few pricing guides and not just one. You'll be suprised to find how one guide will miss something that happened in the issue or won't account for the current demand for that issue. Also be sure to check eBay sold listings. If you don't know how to do this, go in to eBay and search a comic. Then go into filter and click sold and completed to find the actual prices people have paid recently for that particular issue. It's good to set competitive pricing and find that sweet spot that will attract buyers and also have you profit. It goes without saying, but set your price point at what you paid or a little bit more in order to make sure you come out ahead. Experiment with what works and also do research to check to see what others are pricing there books at and if they did sell for that price.
Good luck and as always feel free to reach out to me for any help!