Another well known saying that fits is:
"It's only worth what someone will pay for it"
The price the seller puts on his camper isn't just affected by what he thinks it is worth but also by the reason he's selling it and, to be honest, probably an optimistic opinion that it should sell in no time. As it takes longer this optimism changes to the realism of "I need to get this thing sold so I can (insert reason here) and they are willing to compromise a little to get it done, thus they lower the price.
The opposite is happening simultaneously with the buyer.
The price the buyer thinks represents value is not quite there but their interest has been caught by the features of the camper so they keep looking at the thread every now and then and think more and more about what they might do with this camper if they could afford it. They wait and watch to see if the price will come down at all. Even if it doesn't come down enough, they still get excited by the fact it has moved down at all and as such, they will probably pay a little more than their initial idea of value if they are worried someone else sees it as being worth more than they originally did.
You could almost quantify their enthusiasm to buy and sell as 2 graphs with the sale occurring at the intersection of the 2 lines. (Wow, I just turned myself into a nerd in one sentence!)
Cheers!
Jono.