These three pages are all I ever used to make my item pages (which aren’t as professional as others, but still good). It’s very easy to pick up.
HTML Guide by Envato http://support.envato.com/index.php?/Knowledgebase/Article/View/262
Guide on basic HTML. Very simple.
The Envatitor http://revaxarts-themes.com/envatitor/
Allows you to preview what you are doing. Type in the HTML and you see it in the box below. This is great especially for item uploads, which inexplicably never allow you to preview what your description is.
Envato Asset Library http://audiojungle.net/page/asset_library
Assets which you can use in your item previews.
Additionally, I often like to use photos from Photodune (or other stock picture sites) to enhance my pages. You could also get some logos from Graphicriver, etc… etc…
There are other authors here with far better looking pages than my own though, so I hope they chime in.