haha how useful
but if i say i'm selling something only once, is it considered spam?
The message itself will not be spam if you only do it once and only once. However if you do it in all caps, alternate caps and lower case or use excessive letters, numbers, punctuation or other symbols to "make it stand out", or anything else that is listed as spam in the first post, then it can be considered spamming.
Also if others are posting a lot of trade messages and clogging up chat and you post a trade message it may be lumped with the others as spamming.
So be smart when posting your trade message. If a lot of others are doing it at once it would be smart to wait before you post yours. And there is no need to do anything "special" to "make your message stand out".
Put please be aware this is how it stands now. if trade messages get out of hand again like they were during Jewel Farm trade messages may not be aloud at all.