Indeed, it's my belief it's all about initiating customer contact. By asking the customer to contact you, you're putting yourself in the driver's seat. The customer will only call if they are really interested.
Once the seller posts the price of the coin, the seller is at a disadvantage because
a) the buyer can then shop that price around and find a better deal, thus not bothering to call,
b) they can shop it around, call and say, "but I found it here for less" (total waste of the seller's time who replies, "so go buy it there", and
c) once the buyer has bothered to call, the seller can say, "ah we have it listed for $1000 but we've had it listed for that price for some time. Tell you what, I'll give it to you for $950 so we can clear it out and get more capital to invest" and the seller looks like a hero. Unless the buyer knows grey sheet on the thing is $850.
d) the point of the ads is not to sell out immediately, but to get more customer relationships, get more names, e-mail addresses, phone numbers and credit cards on file. When they list the sale price, even if it's high, a buyer can call in and say "I'll take it", and is more likely to. Then you have disappointed customers who call in to find the coin gone.
e) I suspect sometimes when sellers do this, they have an inventory of similar stuff. "Ah, that one was sold, but I have a very nice one a point higher for this much if you're interested".
In other words, it puts the seller in the driver's seat and gives them a lot more options with serious buyers.
After all that, I don't like it either! I like seeing a number.
But if you want to shop coins, there are plenty of auctions and price guides available to know what a coin should cost.