Hi,
Yes you can use HTML, in the sell form, there is two viewing mode, "standard" mode and "HTML" mode, to write in HTML, you need to be in code (HTML) mode.
Also, note that <p> is no longer part of HTML5 standards (http://www.w3schools.com/tags/tag_p.asp ). You may however use <br> (HTML) or <br /> (XHTML). Since eBay websites are written in XTHML (CSS), you should use <br />