Publish the age of your component when selling (let's discuss)

I've noticed that many sellers don't publish the age of the item or items they are selling on Audiogon.  I believe it should be required that the age or original manufacturing date of the item being sold be clearly stated.  Some do but many don't.  Sometimes you can see the manufacturing date on the back of an item should the seller post a photo clear enough to see it but a prospective buyer shouldn't have to find it out that way.. What is the fear here?  This is especially important when it comes to sources such as DVD and CD players.  No matter how great the CD player may have been at the time, an 8-10 year old unit (let's face it) is worn and the DACs are no longer state of the art. You may think you have a piece of gold but the transports wear and again the DACs are antiquated. 

Opinion now:
I believe that many of the CD players on sale now that are say at least 10 years old are over priced.  To back up what I'm stating here, out of sheer curiosity and with a little extra cash I had, I very recently bought a brand new Onkyo C-7030 CD player for $200.00.  Got it home and in a matter of a few days, OMG, the overall sonic fidelity and extreme detail is produces when playing decently recorded CDs is nothing short of amazing.  Why would I purchase a pre-owned $1,000.00 player when this Onkyo sounds just as great?  I know this is a loaded question and may not be too popular but would love to hear what the members think.  No offense to anyone's machines.  THANKS.  
very little of my gear has a production date listed on it, and I am the original owner of very little, so often have to go on word of previous owner, who is often not sure...and some of my gear was sold for many years...I have that Onkyo you mention and it is excellent for the price, especially since it is readily available for a little over $100.   My Onkyo changer is 15 years old, though that model is still sold new...
Agreed with much of what's said above having bought, and sold, lots of used equipment it can be very hard to know the manufacture date. Also I listen to music and rarely with a stopwatch handy so hours are at best a guess. Do your homework as jmcgrogan suggests also it's helpful when the seller posts plenty of pics. Speaking of which two pet peeves, how can you photograph all sides of a component but not the back. And how can you post photos of speakers and never take the grills off?
Wow excellent discussion.  Love it. Balanced and fair input and great suggestions.  Elizabeth, thanks for the great Google suggestion.  That is surely one way to find out.  One comment:  if you are the original owner, you would (or should) keep the original box and receipt.  Then when you go to sell the item, you can refer to the receipt and date.  That's a practice I follow.  For non-original owners, a little more difficult but certainly discoverable.  I just feel that the seller could expend a little more time to research this information for the buyer.  That's all.    
You can always try to contact the manufacturer with the serial numbers. Most are willing to give you a ball park figure.