Buildings get torn down. Roof needs a replacement and the owner doesn't feel like it is worthwhile to redo the solar install for panels that only have 5 years of warranty left, or maybe they want to replace them with higher power models with a fresh warranty. There are any number of reasons why someone might need to offload otherwise functional solar panels.
Currently used solar panels are a hot commodity, with many groups selling them by the pallet, because 10 year old solar panels are still efficient enough to easily pay for themselves. Very few installations will care about specifically how many panels they want, they just want a nameplate output per dollar figure.
Old inverters might not have a second life though.
