I am seeing more and more of this type of thing. I am sure it's part of the "Secrets to Power Negotiating", as seen on Oprah's Best Seller list -- Never be the first person to mention a price, or you lose power in the negotiation process.
To me, this just sounds greedy. If you are a professional negotiator, or if you are selling a house or maybe even a car, then I understand that there could be thousands at stake, if not millions, so all parties concerned come to the table with that in mind.
But when you are selling a used Centipede trackball or some part off a game that is just taking up space in your garage, why not just come up with an asking price? It can be high or low, that is up to you, as it is your item. But when someone tells me to make an offer, I think "this guy wants to take me for all he can", I get offended, and generally back off and decide not to play.
It just seems like common courtesy to me to set a price that is fair. Then if you want to negotiate, then go for it, but refusing to set a price when it is you that has something that someone else might want, just seems wrong.