The first one isn't necessary - you can assume the receiver won't bother if he/she isn't interested. It would be nice of the person to respond to say "no thank you" but don't be surprised if they don't.
The second version gives the person a deadline. It implies you are holding the item from them for three days. After three days, you will make it available to others. Change it a little, though: If I do not receive a reply within three days, I will assume you are not interested.
("Will" for future here, not "would," and "a" reply, not "any" and no need to use "just."
Student or Learner