I have a blog about PC softwares. If I want to post about software "A" in my blog and I copy it's descriptions from it's official site. Can I do that? I think there's no problem right? Because I may also a bit help them boost their sale
I think it is impossible to prevent duplicate content for product description. It is not that much after all... Adding your own ideas would be great though!
I am not an expert but I would say its not ok to copy their description. Their content is copyrighted. The only question is can you consider your act as a fair use? Is it only one or two sentences together with link to their website? I would definitely prefer to rewrite the content and be on the safe side of the street.
You should ask them. Some places will allow it, others won't. If you find a good dropship company to make your sales with, they often explicitly allow their images and descriptions on you site.
I suggest you ask first and if all else fails, make your own description using some of their key words. The bottom line is as long as you don't copy their description Verbatim you should be fine.