Amazon's sellers sometimes take their time shipping the item but you will get it. Ebay sellers ship quicker as they don't want bad feedback on slow shipping.
This has not been my personal experience. Amazon sellers get feedback also, and
will get feedback that reflects slow shipping. Amazon's rules are to ship within 48 hours, but it doesn't mean everyone follows it. I've only had one thing from Amazon ship slow, and that was directly from Amazon and not one of their sellers. (It went across the country and back, for reasons I don't know.)
Either way, the trick is in reading the feedback, and also in knowing that 95% isn't "good" even if it sounds like it. The other trick on Amazon is if you think you're buying it from Amazon (i.e. not the marketplace sellers listed under "other new & used,") make sure you note whether it says "fulfilled by xyz co." If it says that, don't assume they're a good seller - still look at their feedback. I've had to return two items recently on there by assuming the "fulfilled by" items were like getting it from Amazon. It's not. Sometimes the small marketplace sellers are better.
(BTW, I ship in the same amount of time no matter where I sell it. I know of others who do the same.)