This feature was actually removed by eBay a few years ago, no idea why they haven't removed the checkbox in messages, but it's pretty typical of them to just get rid of the important (to them) bit. Initially the feature was replaced by a Q&A section similar to other sites, where members could ask a question via a listing, which was posted publicly, and then anyone who'd bought the item in the past, and / or multiple sellers of the item (sometimes not even the original listing's seller), would be emailed and prompted to answer. The Q&A would appear on all listings for the item in question (identified through product IDs)
This didn't last long - lots of confusion from all members about why they were being emailed and asked questions etc. Quite a few thought they were scam emails. And of course buyers interested in the item would think they were asking the seller of the listing they were viewing so there'd end up being questions about postage and other things only relevant to one seller, who may or may not receive notice a question had been asked. Once that got canned, eBay didn't replace it with anything.
For BIN listings, you can at least edit in info that seems useful if pointed out via a question. For auctions that can't be edited, I think you can still add to the description yourself (nothing about the original listing will change, but a section is added at the end with the date / time it was added, you can only add text this way).
