I've started to implement this feature on the site I'm working on, and wanted to know if it's a good thing to do.
Basically, just like any other site where you can post your ads, you have a field to include the description of what you're selling. This is often just plain text. Would it be any better if there would be markup involved? I would think of things like
### Header ### and such, so the descriptions can be formatted better, if someone has the time to (and feels like) do(ing) it, but I wouldn't have to worry about urls being posted, or images embedded, etc.
I think it would be a nice little extra added to the site.