Hi... above responder is correct that you need a store to do that but read this link for some information to help.
http://reviews.ebay.com/How-to-Turn-Off-amp-On-your-eBay-Store-for-your-Vacation?ugid=10000000016514...
If no help then you could learn some HTML codes (that are free online when you do a search) that could help you and if you have bids on your listings it will not be able to add.
If you do not have many listings you can revise in the HTML tab to add this code at the top od your discription.
<marquee behavior="slide" direction="right">XXXXXXXXX</marquee>
In the section of the code above between the >XXXXXXX<... you can type in something like" Going on Vaction between I will be away from Jan 13 to 20 and will send your item on XX date'.
Below is a HTML Practice Board to help you to fine turn it:
http://www.practiceboard.com/