There is a seal that must be broken to move the odometer back. However, it's pretty hard to disguise a car with substantially more miles than shown. My concern with any car of that age with that few miles on it is that it has been sitting for a long time. Cars don't really like to sit without running. You can't turn the odometer back using an electric drill or anything like that.
Actually, I'd rather by a 2006 with 50,000 miles than with 30,000 miles. If it was driven every week or two then it's fine, but if it wasn't driven for months on end, then that's a worry. Your best shot is to have an independent Prius mechanic check it out.
Note that the Prius has a great reputation for reliability, so 30,000 or 50,000 is really just like new.
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