As a person who has made hundreds of trades on here, I actually find it to be really rude. If you post in a thread wanting something then you should be available to trade right then and there (or in a few minutes or so). I know personally that I usually wont hold any items or villagers for a person who isn't available at the current moment. I understand that emergencies (or other things) can happen that make it impossible to trade at the moment but if you are at school or at work then you shouldn't post to trade just so you won't miss out on items. I know I am sometimes on the forum and I can't get on my ds, but when that happens I don't post in buying/selling threads I just let someone else get the items...there will always be another chance to get what you want, there is no need to be selfish about it.