Write your own unique descriptions for each product on your site.
Seriously, that's all there is to it. There's no magic, just the hard work of actually writing your own unique product descriptions.
If you try to take the easy way out by cutting-and-pasting from vendor product descriptions or using the product descriptions included in vendor product feeds, your product descriptions will almost certainly be duplicates of other sites that are selling the same products you do and which have also taken the easy way out.
Even if you write your own product descriptions, when you cut-and-paste product descriptions from one page to the next, you'll simply be duplicating yourself
. (Referred to in some circles as "shooting yourself in the foot.") If you have products that are so similar you can use the identical description for each (perhaps only changing one thing, such as color or size), consider combining them into a single "master" product with multiple variations.
Think of it this way: there's fast, there's cheap, and there's good. The problem is -- you only get to choose two.
Fast + Cheap = use product feeds or cut-and-paste. It won't take long, it doesn't take much work, and it will subject you to duplicate content filtering with the search engines.
Good + Cheap = write your own unique product descriptions. It won't cost you anything out of pocket and your descriptions will be unique. The downside is that it will take some time and effort, especially if writing doesn't come easily to you.
Fast + Good = hire a talented writer (or two, or several) to write unique product descriptions for you. It won't take much of your time, but it will cost you something out of pocket (especially since you can't hire really excellent
writers for peanuts).
Your time, your money, your choice.