I like the direction of the suggestion.

...or restrict such nascent valuation threads to e.g. the classifieds section (some new subsection thereof) as we see in place elsewhere. One-stop, one-thread shopping for the discussion, and then 'for sale' post.

Idea being that we may still want to assist folks in valuation/knowing what they have, as it promotes them to sell rather than stuff them under the proverbial mattress. Just without the clutter/distraction in the main forum.
(not speaking to the on/off topic part per se)

That said, there would be some consideration of the cost of the traffic, sans donations... but perhaps worth to subsidize it by directing the traffic here and making it easier for folks to find what they want to buy.