b.A) I received an email and it ended with this sentence: Please let me know if there is anything else.
Do I need to reply if I do not have anything to reply? Is is rude if I do not reply anything because I do not have anything to reply? If a reply is needed even if I do not have anything to say, what should be an appropriate reply? Should I reply "That's all"? In this case it would depend on the subject/reason/tone of the exchange, but in general in that example and without knowing the context, I would not think a reply is necessary.
B) Sometimes I see this at the end of the email or article:
B1) Let me know what you think. In this case as the sender, I would expect some reply, and again without knowing the context, I might respond, "I agree" or "I can't think of anything else right now, but if I do later, I'll get back to you".
B2) Let me know if you are interested. As the sender I would expect a response such as "I don't think I am interested at this time. Thanks".
B1) If I do not have any opinion, do I need to reply? Is it rude if I do not reply?
B2) If I am not interested, do I need to reply? Is it rude if I do not reply?