If the collectible is from a limited edition or it's been discontinued or retired, say so. Note if you have authentication-a professional appraisal, letter or proof of authenticity, and so on. List any other details, attributes, or flaws. If your descriptions are detailed, try to keep them as brief as possible. Some buyers feel overwhelmed when they see a large amount of text.
Buyers respond better to text that's been broken into sections.
See eBay's Common HTML Tags page. If you add other HTML embellishments to your listing-animation, music, wallpaper, and so on, it will add to the loading time. Most buyers prefer a listing to load as quickly as possible. In general, you'll sell more if you concentrate on making the text quick to load and easy to read. As tempting as it may be to type all in all capital letters, don't.