First, I think of the best features of Auctiva is that its free! How can you beat that!? What I like mostly about it is that it has a> free image hosting and 2> great community support.
As for your other questions, I think to link websites you simply need to start networking and finding businesses that are somewhat related to you and offer them a link exchange. You could also post in various business forums that you are seeking partnerships for that purpose.
As for adding signatures, click on your name > click on View Public Profile > click on View/Edit Complete Profile, scroll down to the bottom, then edit your signature and save.