I assume you want us to post them? Dealers who charge for top quality coins, but then when it is time for the dealer to buy, they won't pay for quality.
Are you asking what a coin dealer's pet peeves are, or are you asking what our pet peeves about coin dealers are?
How about treating customers like thieves? I was once told by a owner to sit next to him and drop my bag so he can see what I was doing.
When they call me to their table, and then ask a load of questions. I makes me very uncomfortable. Also when they treat me like I know nothing about what I'm buying, regardless of value.
"I don't have it but I can order it for you. Leave your name and number and what you want and I'll call you in a week when it comes in." Call him after 2 weeks of no call and he says "I don't have time for this, I got customers" and hangs up. I'm quite sure he's getting their names and phone numbers for what they want only to then flake on them also! So my peeve is LCS's that don't know how to run a business and would rather run it like their personal club house.
In general I don't support pet dealers. The animal is usually ill treated and sickly, and of questionable pedigree and many come from unlicensed sources. Those are my top peeves.