How do you avoid scammers on Ebay as a seller.

I am having from time to time buyers coming back to me claiming that I have sent them faulty goods.. this is despite that everything I send out is in pristine condition and is well packed and tracked.  Hmmm had one the other day who was fairly abusive, threatened negative feedback which she has now posted on my site and I have replied to.  She refused to send goods back despite the fact I offered her the return postage and a refund if I thought she had a case.  Personally speaking, I packed the item well, it was in good order when it left here.  Had a few of these sorts of folk, and I think there are people out there at the moment trying this on and then refusing to send back goods.  Not the money here definitely not ,as the item was not worth the effort really.  However, the principle of the thing is at stake here and there appear to be more of these people coming through Ebay.  I suppose you can't have everything and you have to expect some problems but are any others (sellers) having this trouble out there?????  I am a very ethical person and expect people to treat me with respect as I respect them.....am I expecting too much????