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

I agree with "put it on ebay" where the audience is very large. You just might find someone with interest in such a camper. Everyone else will see it as an old, worn out camper that represents a huge expense to restore to useable and not worth the effort. It will be a specialty item and take a special type person to show interest. One man's "treasure" can often be another man's "trash".