Creators of pony items often enough put them up for sale on sites like E-Bay or Etsy or their own business sites. Could a tag be useful here?

I think it would, though most of the time we've just done this through comments since they're only up for a limited time. Searching "for_sale" or something wouldn't work if it's a one-shot item like a plushie. :(

"for_sale" "auction" or just "auction" would work out pretty well for that though.