How much are my mavics cxp 12 worth?

Regardless of where he's selling them- he's still advertising a price.
Assuming he hasn't just found the rims on a tree or made them, then he knows they have a set value- because he bought them for a set amount.
Regardless of this fact all sales have a base price below which it would be stupid for the seller to sell, because the item will have greater value to the seller than the buyer.

Dishonesty in selling is not pulling the advert if a price is not met. The seller is under NO obligation to sell.
In fact he can even raise the price. Fuck, as a seller, he can do what the fuck he likes.

If a buyer accepts a price then he and the seller are then, and only then obligated to trade, otherwise, then it becomes dishonest.

simple economics in fact dictates that things are worth exactly:
A. what someone is willing to pay.
B. what someone is willing to sell at.

the balance of the two dictates the price, and hence we get the idea of supply and demand.
Dumbed down, and poorly written on an internet forum

He's not advertising a price. He's asking for a guide price after a year of use, in which time demand may have changed.