Trade Listing instructions? help please

I don't believe there's a true guide for it (anyone able to correct me on that, please do ) but the too few characters would indicate that you're just not utilizing the minimum fields.

Generally and a bit simplified speaking, a trade listing is a 5-step process:

1) Go to trade -> new trade in order to start a new listing.

2) In the top line (red below), enter a title/subject for your trade. Excluding summary titles for rares and crafted items, this should generally be the actual exact name of the item you're looking to buy or sell. As you start entering said title, the site will suggest an item to match if there is one. E.g. if you start entering "hellf", the site will load suggestions and offer you for example a "hellfire torch" to list. Clicking that suggestion if correct is your easiest way forward as it pre-selects most applicable tags (see step 3).
For anything that is a "common item", be that a base, a rune, a unique item or the like, always just go with the suggestion provided as it makes sure folks can find it correctly - if you don't and only enter the name and then forget to set things like the rarity tag for uniques manually for example, there's a decent chance folks won't find your listing when they use the suggestion to search because their search will include the tag.

3) Select the applicable tags for your item (yellow below). That's things like where you trade (game mode, platform, ..) whether you want to buy (WTB) or sell (WTS) and various bits of information about the item. Look through those tags to get a feel for what's available and just select the applicable ones for your trade. Generally, for something that is clearly defined (e.g. a unique item), you don't need to bother selecting tags for individual stats (except perhaps the class tag for a hellfire torch for example) but for things like a rare or crafted item, where stats could be just about anything, selecting the correct stat tags (res, life, mana, ...) helps people actually find your listing.

4) Enter the actual trade information in the body field (green below). This is where you put in the text that folks should see when they click on your trade to view it. Typically, that is things like a brief description/recap of the exact stats for items with variable stats or a screenshot of the item itself to make it easier (especially convenient and common practice for rares/crafteds again) as well as your asking price (for WTS) or offer (for WTB).

5) Hit create trade listing to finish your listing and submit it.
At the very least, a listing requires 1 character in both, the subject and the body field. The error you are seeing stems from not having anything in the body field (the green one above).