I'm actually a little taken aback by a post like this to be honest...
Tyflier already said this, but you aren't actually OWED anything by anyone. Would it be NICE to receive a response to information you give someone? Well, sure - everyone likes feedback, even if its only to say "Thank you but I've already purchased a snake" or "I've decided to go another direction". Yes. That would be very nice. But is it "impolite" or "rude" or even selfish to not do so? No.
However, I find it EXTREMELY impolite and rude, and even out of line, to start a thread 'asking' if someone is being rude by not responding to you. For one thing, you are referring to a person who is a member of this community. Sure, you didn't name them by name - but obviously it isn't hard to figure out who you are referring to. If you want to discuss this kind of behavior and openly flag someone as being rude - maybe you should do it at dinner with your buddies instead of in a forum where they are likely to read it?
Also, there are plenty of times that I make inquiries into things, such as snakes or kittens or coffee tables or what have you. And sometimes either A) I don't see the response. B) I've moved on from that desire and/or C) I just don't like what was offered to me. Am I REQUIRED to inform each person that responds to me my intentions with their item?!? No, I am not. In fact, I will flat out refuse if I'm given an attitude from the seller. And if I found one someone was calling me rude, selfish and inconsiderate solely based on the fact that I didn't comment on a link they gave me - or replied after numerous emails and PM's because they wanted me to buy something of theirs? I would be furious actually. ESPECIALLY when someone is explaining to you - over and over - that the person you are hounding for a response most likely hasn't even seen all your attempts.
ALSO... Your response in the original thread was simply a link to your classified ad. No personal message to the OP, nothing to indicate you were being friendly or personal. Just a link. I would feel even less compelled to reply to that sort of message and am even more surprised you seem so positive you DESERVE something from that post.
Its shocking to me. I would be surprised if anyone that has had the chance to read this thread would be even remotely inclined to inquire about your snakes - or respond to any link you offer up. Just my opinion... (and a few others it seems).