Spamming is the sending of unsolicited and unnecessary messages. While we understand the need to message users to discuss trade, there are certain messages that are plain annoying to the receiver of the message.
One way to tell whether what you're sending is spam is if you find yourself copy/pasting your message to multiple users or repeating the same message over and over.
Sending links to your trade page or forum trade thread to users who have not asked for it.
When you are searching for a specific pokemon, and you message a bunch of users inquiring about it. If you message 10 users when you only need one pokemon, you are wasting the time of 9 people who you have no intention of trading with. Be considerate of other people's time.
What you can do instead is to politely message no more than 2 or 3 of the users you were considering based on who was online most recently. Consider showing some interest in something else they may have for trade.