On RP servers, even with trade I believe not being an In Character Channel officially. you often get In Character advertisements. These are often longish, and at times can feel a bit dense and spam like. You only need 2-3 people Dumping a 4-5 line profession advertisement to cause my chat window to scroll at least.
There are people that use standard ascii art methods to make the advertisement more attractive, the least annoying of which I think are basic hi-lighting attempts by using underscores and dash's etc. You could use the standard 'flower' ones if you were selling weeds for example:
Has been used for as long as I can remember for text only flowers. Variations abound. There are also ways to get the extended characters out of WoW. Things like • § £ ™ º ¢ etc, these require you to know the extended codes for them - which depend on your operating system. But I will caution against the use of those, as well as any formatting that relies on the length of a line, tabs, or the like. Anyone that has installed any chat mods, or has customized even the size of the chat box may well not see what you wish, and more to the point may see something that looks poorly done. Same holds true if the font is replaced and the replacement font lacks the extended characters mentioned above.
Personally my advertisements are of the following format.
[Jewelcrafting] Currently in the Center of Dalaran.
Substitute the location and trade as approprate. I err on the side of a clean informative post, and if I am demanding a fee (I accept but do not force payment, so I don't normally get into it) or have a special in demand pattern I might mention it.