I am planning to look at a 2010 Prius at a dealership which hasn't been sold for the last 300 days. What are the implications to the battery if its sitting there for a long time?
welcome! deduct the price of a new 12v, hybrid battery, egr circuit cleaning, and possibly brake actuator.
It will almost certainly have damaged the 12v and a new replacement should enter the negotiations. I doubt the car sat completely idle for 300 days. Test drives and simply moving the car around on the lot dozens of times required it to run over that time span. Low use has certainly reduced the life expectancy of the expensive HV (high voltage) battery versus a daily driver. There's no easy to way to determine its health without some tools: smartphone, apps, and an OBD transmitter.