Ask any questions that you have before you bid or before you click on buy it now. You are committing to buy at that point. It is usually pointless to ask a seller if something is authentic. It is illegal to sell counterfeits so even if they are not sure of authenticity, there is a good chance that they won't tell you the truth.
As far as the unpaid item strike, as long as you just just have one, it shouldn't affect you much.
You are buying a lot of high end purses and some of them are from sellers with less than stellar feedback. It is a very good idea to go to the ebay.com shoes, purses and fashion accessories forum to ask if they think the bag is authentic. Do this before you bid or buy.
Here is the forum url. http://forums.ebay.com/db2/forum/Shoes-Purses-And/1000000009