It looks like the 3 main points brought up are photos, description, and shipping cost. My thoughts:
I think the photos really need work. They're reasonably clear, but very, very dark. More light, and/or use a flash! I would also use an editing program to improve them, along with putting them on a neutral background, as others have suggested. If you can't afford a commercial program, G.I.M.P. is freeware (they ask for a donation), and while it's not exactly intuitive, it's very good and does a lot. I've been using it for about 2 years, and am gradually improving my photos as I learn more about it.
The only reason--in my opinion--to use Priority Mail is for speed, or to get the cheaper flat rate shipping for small but heavy items. The books should definitely go Media Mail, and the rest should go First Class unless they're 13 ounces or heavier. If you need cheap packing materials, try uline.com. They are very reliable, and stuff comes in 1 to 2 days.
I agree that the descriptions are easily overlooked. I like template programs like Auctiva because you can highlight the pertinent information in a color block, and it's faster to list many items in a session when the basics are already filled out. It's been years since I've used an Ebay program, but that was available with Seller's Assistant Basic before it was discontinued. I assume they still have something similar.
One more thing--don't underprice your items. I see this a lot with new sellers. Figure out your cost basis, including sales commissions, Ebay and Paypal fees, shipping and shipping materials, and price accordingly. It may be that some items simply aren't worth the bother. On others, you might be better off selling at a higher price. A quick sale isn't worth a thing if you are losing money at it!