How do you find out how much to sell an antique pop-up for?

Here's something similar-sounding that's been for sale for a month at this link

According to the ad it's been in dry storage for forty years. It's so pristine/complete that it even has the original real mattresses/canvas sleeping bags, not to mention practically unused kitchen/campware including some GRISWOLD cast iron, and even the original turquoise webbed aluminum chairs! Link to more pics

Price: $1800.00