One thing that really gets my goat is people who can not be bothered to communicate properly. If I send someone a trade offer, or a message which may take a matter of seconds to reply to, I expect a reply. If it's a short "thanks but no thanks", no problem. But hearing nothing? The same applies in maths trades, when I send a mail asking if you have received the item I sent - and I hear nothing. Call it bone idle, inconsiderate or just downright rude, this is clearly not acceptable.
I have a short list of names (now up to 7) of people who fall into this category, including instances when a trade is agreed and the seller changes their mind, providing lame delaying excuses. I will flatly refuse to deal with anyone on this list. I really can not be bothered with them.
References - if I am a seller, and payment has been received quickly, I will post a reference upon receipt of payment. If I am a buyer, I will not post anything until you have done the same, effectively saying you've received payment.
Maths trades - I will post a reference as soon as I receive an item, and upon either a reference or a mail confirming receipt I will post a reference when I have sent nothing. No communication in this case leads to a negative reference.
Simple courtesy and manners are worth the world, but do not cost anything.