this is a normal way of trading business. they source the books and you got the cheaper price. it's ok.
unless you can also buy from dongdong,which indeed you can next time.
there are also many many people source the things from taobao and sold in yahoo or retail shops in hk. then you cannot say that is not honest business.
i say the toys i bought in taobao are sold hundreds of dollor in some online shops in Hk.