I was looking thru posts and I can't find many complaints about the original AD. Most understood that you could get misc items, some nice items to have to help with events but not an upgrade to a recent event. I guess you miss us complaining. Some actually liked the little items too, even decorations. So, why not just add recent event building upgrades, make the minimum somewhat normal. Normal, I wouldn't expect 100K, too low but people will bid it up for sure. Then leave the rest alone. Then it will be like a real auction. If you asked me what would be a fair starting bid for a metro selection kit, low ball - 500K, no more than 1M.
In 6 months, the metro kit minimum should reduce too, it is getting older, less value.
Instead of keeping the buildings you intend to upgrade a big secret, like hidden in the briefcase of the US nuclear codes, why not gives us a clue.
For instance,
cherry set - we do not plan to do upgrades to any buildings in a set
Pirates Hideout - we do not plan to do upgrades to event buildings with chains
Winter Chocolatery - event buildings with options for the final level, we may have an upgrade in the future
Any other event building, like abandoned asylum, is fair game for an upgrade
Then players can decide what to sell, take risks if they want, maybe they want to bid on the possible recent event kit, but they have to be able to make some coin. You can keep the current "sell" values if we could make educated decisions. If I could only bid on 2-4 recent kits in a year, I think that is fair, I am not saying to give it away.