Yes, you can only add "seller is away" if you have an eBay store.
If you are only going out of town for a short time, you can use the bulk edit tool to adjust the handling time on your listings. For example, if you have the handling time set to "next day shipping", but you will be out of town for 3 days, you can choose a longer handling time rather than end your listings. If something does sell while you are away, just send your buyer a polite message that says something like "thank you for your purchase! I am currently out of town, but will ship your item as soon as I return home on Tuesday."
If you are going away for more than a couple days, it would probably be best to end your listings & relist after your trip.