I don't see anything wrong with mentioning an auction in a forum, or contacting a seller to let them know they have something worth more than an "old piece of junk."
There are a more eyes shopping out there than you might imagine, if you only read the posts in a forum like this. Within a 5-mile radius of here, there are two different collectors who own world-class radios that make my feeble collection look like dog doody.
If those guys read a forum like this at all, they are remarkably silent, but I can testify that they know how to scour eBay like hawks. eBay also includes international shoppers who may have no interest (or the language fluency) to participate in such a forum.
Visitors to my website often ask questions about the value of whatever they own. Most times, it's something of little value, but if it's something more significant, I give them my honest guesstimate rather than try to steal it for a bargain price. I already have too much stuff, and life is too short to spend it trying to flip things on eBay.
If you want to "call dibs" on an item so that other forum members don't compete with you, I don't have a problem with that. But there are other silent shoppers out there, from Asia to Europe, and if one of them spots the item and decides to outbid you, that'll happen, whether or not the seller understands the true value or brags it up in an auction description.
Just my $0.02.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html