In my humble opinion i think its slightly rude IF you simply ignore a private message in which i politely try to begin a negociation with you and have to ask in your public thread to get an answer that IMHO you could just have replied my Private message with.
Somehow it must've been a too big effort for you to do, since you did not choose to reply to my private message.
Or maybe you saw my private message and could not reply in the situation you saw it, and forgot to do it later.
I am not judging you, because i dont know the story behind why you never replied