I'm thinking of selling my sisters most valuable books on ebay without asking her permission.
Oh no, you shouldn't have done that at all. This is not right to take her stuff!
The line between right and wrong is a very thin one
No, the line is very thick because this is simply stealing and reselling things that aren't yours. That is obviously a crime and there is no excuse for it. You need to work together with her to find things she does want to sell off.
Nah, she can just suck it. I'm doing what I want with her stuff.
Stealing can get you in a lot of trouble. Why would you want to upset your sister?