Should listings include the age of the item?

I typically include that info and if I do not the question can always be asked once an interest is shown by a prospect.

Did you experience a problem with this?

Condition of the unit and use may be much more telling than the age. Buyer should always ask as many questions that may be of concern. Seller should always be ready to answer any and all questions or clarify particulars of a item up for sale.

Pricing should be determined by market value at time of sale. As seller may or may not gotten a "good deal when they made the purchase. How be it rare, some products go up in value. Listing the original MSRP if known at least provides some base value. I often assume original sale was 10 to 20 off list price. That is the best deal most are likely to have gotten. Spending a few dollars to obtain blue book value or other research keeps one from paying too much.

What if the age of the item is unknown? Granted, many model numbers change every 2-3 years so you can somewhat figure it out, but many do not. I have bought used gear where the seller did not know how old it was, how am I to know how old it is then when I sell it?

As for your #2 concern, when posting an ad, the Audiogon system ask that you post a MSRP price (if you know it). They do not ask that you post the price that you actually paid for the item. It is up to the buyer to understand the market.