Whatever it cost new is irrelevant now, it's a sunk cost. It doesn't matter if the buyer paid $5,000 after rebates or $50,000 with no rebates. I would definitely confirm that it's owned and not leased. I would think the owner should have an invoice or something from the installer showing them paying for it.
I would be concerned with how large the system is, i.e. how many kilowatts is it rated to produce, and how much electricity it's actually producing. Most importantly, how much it's saving the owner. It could possibly be providing income if there's net metering and the average daily output is more than the home is using.
All things being equal would you pay more for a house that had an average $200/month electric bill or one where you received a check for $50/month? In this situation that's $3,000 a year a buyer would be saving. That's got to be worth something. Even if you just look at the present value of that income/savings over 10 years with a 10% discount rate it's about $18,000. If you drop that discount rate down to 5% (roughly what the buyer would be paying to borrow more to fund the higher purchase price due to this cost savings) it's about $23,000.
There are a lot more sophisticated ways of analyzing it with more detailed spreadsheets or trying to find sales data, but this is just a simple check of reasonableness that you can do in a few seconds if you know how much the system is saving in electricity on average each month.