I've seen this discussion come up on just about every internet community I've ever been part of.
The longer a community is around, the more it tends to crop up.
I think it is much more of a problem in larger communities with less niche items. Like you said,
@Troy, this site and possibly one or two others is pretty much it when it comes to weber collecting.
Frankly, until about a month ago, had you told me that there was a market and rabid fan base for old weber grills, I would have probably laughed at you. I say this as someone who has believed (and stated many times over the years to my wife and other forums) that if there is an item out there that can be collected/coveted/hoarded by more than one person, there is almost certainly a forum out there somewhere that caters to them.
I agree with
@terrymo, you can try to put some guidelines/barriers in place for it, but if someone has no interest in contributing to the community they'll just post pad somewhere (usually OT if the admins haven't disabled post counts in the OT sub) or post BS answers in other threads until they can post their ad and leave.
In the situation of this specific community, however, I think the problem is less of a concern. Weber grills are such a niche collectible, most people are not aware of the idea there is a value for them. If anything, the old ones suffer from the Antique Roadshow/American Pickers hangover effect that is plaguing every single flea market and yard sale I've been to lately. Everybody thinks their junk is the diamond in the rough and is worth a premium price. I can't tell you how many fully rusted old tools I've seen with no significant value lately that are $20, $30, $40 each. Before reality TV made people think their garage was a retirement plan, you could get an entire toolbox for $20. I just saw a WSM on CL for $325, and the guy is priced "firm" for it.
The flip side, though, in this arena, is that most of you seem very well connected to each other. It is doubtful too many people float in and out of this community without getting to know them to a degree, due to its size and uniqueness.
Just my $.02, whatever its worth from a guy who is new to this community but not new to communities like it.