I’ve used that option quite a bit, for everything from books to replacement blender jars. 80% of the time the item comes quickly, i.e., within a week and a half. The other 20% may be slow–up to 3 weeks, but I’ve never had and item not come.
My husband sells books and videos through Amazon Z-shops. It’s similar to buying from E-bay. The sellers are rated with stars and anyone can see the feedback.
If you know the seller’s name, you can check on their z-shops page. My husband’s page is: http://www.amazon.com/shops/calsbooks/ (just so you can look and see how it works). Or just click on the seller’s name under the item you’re interested in and you will see their feedback.
As always, use good judgement. And if you order something and it doesn’t arrive in a reasonable amount of time, contact the seller first. But be aware that once it’s in the mail it’s out of their control. We’ve had books take over a month to get across country due to the post office - we mail them out within 2 working days, then they are out of our hands. It’s frustrating for all parties.
I use that option a lot and I tend to stick with sellers with high feedback ratios (98% or better) with lots of ratings (meaning lots of people have used them.) With that little policy in play, I’ve never really had a bad experience.
I bought a textbook from one of them and a DVD box set from one of them, didn’t have problems with either. I do agree that you should stick with the sellers who have numerous positive feedback though, probably safer all around.