Remember to be polite, use non-inflammatory words. If you do not do this the message will be mistaken as your own. This could also be embarrassing for you should the original author discover this and point it out to you online. Sometimes an online environment makes it easy to do something you would not ordinarily do. In general it is customary to quote pieces of other messages within the discussion forum itself, when you are responding to it.
Online discussions are sometimes like that.
However, if you are publishing the message in some other way, for an article for example, copyright law requires you to ask permission of the author prior to use. Yes, but politely. If you are an experienced online user, think back to your initial experiences and remember an error you may have made. Sometimes we are not aware of all the rules of the road. If you do notice an error, politely and privately alert the individual to it, perhaps via email.