I would generalise this a little: The problem with buying a complex item is information inequality. The seller knows more about it than the potential buyer. When inspecting the item, the buyer therefore looks for signs of hidden defects or for evidence that the item is in good condition overall. In this way, cleanliness becomes a proxy for the general condition of the object. Similar mechanisms are at work elsewhere too. Take job applications, for example. A sloppy application is a red flag, even if the applicant’s references are excellent.

OT: a friend of mine recently bought a house. They were surprised to learn at closing that the seller had financed their solar panels and that wasn’t apart of the home purchase.

So they had to negotiate who was going to pay off the solar panel, and this all had to be completed before home sale.

Exactly the same reason you should probably clean the rest of the house when you're selling it.

Theoretically, the price of the house before cleaning should be equal to the value if the house after cleaning, minus whatever the cleaning costs. In reality, you'll get much less for your house without cleaning it, because human emotions.

Actually in some places panels devalue a house because some people consider them an eyesore

A patchwork of eg new paint is a good signal they're covering something up too

Also you don't need anything fancy when washing them, their surface is just glass.

I agree. Doesn't matter what you sell, you will get a higher price if it is cleaned and tidy when presented to the market - because the kinds of people who pay higher prices are looking for clean and tidy things.

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