I see a lot of ads on CL and I've placed a lot of ads myself. We can't all be ad copywriters or pro product photographers, but if you want your item to sell, here are a few tips:
1) Take lots of photos. Take them from every imaginable angle. Ads with lots of photos are more likely to sell than those with few or none. I'll say that again for those people in the cheap seats: NO PHOTOS = NO SALE. I can't believe I have to mention this in a photo/video group. Be sure to put the best photo of your item as the first featured photo.
2) Make sure your photos actually show the item. Get up close and fill the frame. Take them in bright diffused lighting so the details can be seen. Don't photograph in bright, direct sunlight. The harsh contrast makes it difficult to see details in the shadow areas. Luckily, here in Seattle, bright sunlight is rare.
3) Include photos of the equipment in use or at least turned on. People want to know it works.
4) Photograph your items with a plain background. It doesn't have to be on white seamless, but the fewer distractions in the photo, the better.
5) Price your item realistically. Check online stores used item prices. Look at other CL ads. Remember, used items at online stores usually come with return privileges and/or warranties. Your item most likely doesn't.
6) Don't compare your price to something new unless it is also new and never used. Once you've opened that box, it's considered a used item regardless of condition. A used item is subject to market values despite what you may think it's worth.
7) Grade your items realistically. A camera with 50K on the shutter in NOT like new. Don't claim what isn't true.
8) Respond to inquiries in a timely manner. This should be obvious. If others are selling the same thing, buyers will move on if you don't respond.
9) Check your spam folder frequently for responses. Email from craigslist often ends up in spam.
10) Anticipate what questions a buyer may have and answer them in the ad.
11) List what is included in the price, and what isn't included.
12) Write about the item you're selling. What makes your item unique? Is it extra clean? Is it rare? Does it include additional parts? Let buyers know.
13) Be honest. If there's something wrong with the item, let the buyer know. Why waste people's time if after inspecting it, the buyer finds the ad misleading. Why? Because...well, karma's a bitch.
14) If you're NOT willing to negotiate price, say so in your ad. Many people look at the price as just a starting point.
15) An ad that runs for 2 weeks without any interest is usually poorly written or overpriced. Make changes before you renew.
16) Be sure to categorize your item correctly. Photo/video is for photography and video equipment, not for actual photos, movies, or DVDs/VHS tapes.
17) Don't spam! It's okay to crosspost if you think your item fits in more than one category, but avoid posting multiple times in the same category.
18) Be smart and only deal with local buyers. Be careful who you give your email and phone number to. Scammers have many techniques to cheat you or hack into your email. Don't fall for it.