I can respond as someone who has bought gear from the classifieds here (although I have not sold it). What I look for:
1) Clear description, including (as applicable) things like size, year new, service history, condition (including flaws), and price. Photographs are immensely helpful. If you have taken good care of the item, tell me how (soaked after ever use, stored in climate-controlled room, etc.).
2) I like to see what payment types will be accepted, whether or not the price includes shipping, the location of the item, etc.
3) And finally, although it's not at all necessary, sometimes it's nice to know why the item is being sold. Again, not necessary, and if it's "none of my business" that is fine; but on the other hand, something like "bought this for my spouse and it didn't fit," or "I loved this X but I decided I wanted Y feature it didn't have so I bought another model - same brand" really does help when as a buyer you are trying to figure out what you might be getting. I guess it humanizes it.
I typically don't respond to ads that don't list a price, even if it's something I'm specifically looking for.
Blue Sparkle