Adams Trail-a-Bike Folder One tagalong bike. Details here:
https://www.trail-a-bike.com/products/trail-a-bikes/original-folder-one/
https://www.trail-a-bike.com/img/manual/08.trail-a-bike-owners.manual.en.pdf
We used this until our daughter was eleven years old. At around 70 pounds, she was still within the official weight limit, but I felt like she should be using her own bike all the time, instead of relying upon Daddy (i.e., me).
Condition = been used a lot, but never abused, and abuse probably wouldn't make any difference since this thing is so bomber. (Although that also means it probably weighs as much as a couple of my own bikes combined!) Comes with all four seat post adapters for the hitch.
I have this posted for $95 on Craigslist and Facebook but would be happy to sell for far less to avoid dealing with any weirdos there. (As opposed to my fellow NCC biking enthusiast weirdos!)
A couple important points that may or may not be obvious:
1. Don't even think about trying to attach this to a carbon seatpost!
2. I would not recommend this to your neighbor or colleague who is a weak shaky cyclist, as the adult in the lead needs to a solid rider in order to be prepared for the occasional unbalancing gyrations from the tagalong.
Can also include a Specialized Shuffle bike helmet, size 50-55cm, in excellent condition, no crashes ever.
Note that the riding pictures that look like they were taken somewhere else were indeed taken somewhere else (Boulder), with a rental tagalong, but same model. (Commentary from our daughter to accompany those pictures = “I pedal only when we’re going downhill.” & “Are you sweaty? I’m not!”)