You may think that font point size is a trivial matter. People can adjust point sizes with their own browser settings or do it manually when they get to a page (I do this sometimes by using CTRL-[scroll wheel]) However, not all Web surfers know how to do this. You can´t assume that they do. And with printed materials, you only have one chance to pick a font.
Choose wisely. Size does matter. In Colin Wheildon´s excellent book on layout and design, Layout & Design: Are You Communicating or Just Making Pretty Shapes, the research data is clear — fonts sized 10pt to 14pt are preferred by most readers. 11pt and 12pt are most preferred out of those. Not surprising, but it´s something to consider when you want people to read what you have to say.
Whether it´s a post card you send in the mail, a flyer you stick in an eBay package, or the description you write on an ecommerce or eBay page, your main objective is to communicate. Why not make it easy on the reader and start off on the right foot? Readabilty and comprehension are key communication foundations. Build on those with compelling, benefit-oriented copy (or "what you stand to lose" copy), clearly worded offers, consistent closing, and transparent policies.
You will sell.