There are always bad merchants. It's what you'd have to expect in an MMO. Not everyone behaves rationally. Personally, I don't think I've ever been a bad customer or seller. I always take offers seriously, even if they aren't quite what I want. It's always good to have a backup buyer in case you absolutely cannot find someone buying at your asking price.
One thing I noticed you didn't mention is that some people want cash only. In my opinion, cash only trades in Mabinogi sometimes close the door to really good deals. One time, I was selling a Karis Wizard Suit a few years ago when it was still pretty rare and expensive. I was having trouble selling it for the gold price that I was asking at and I also wasn't too fond of item trades at the time. However, I gave this one guy a chance who wanted to meet up and show his items that he wanted to trade for my Karis Suit. When he started shoving things into the trade window, I was amazed. Although it's been too long to remember what he actually put in the trade window, I can clearly remember that the entire window was full of items, each worth at least 1/4 the cost of the Karis. I thought he wanted me to pick and choose a few of the things, but he actually wanted to trade THE ENTIRE GOD DAMN WINDOW just for my Karis. In the end, after selling the goods from that trade, I think I ended up with twice the gold I was expecting to get initially.