I learned of it in 2021 selling some upper soft doors. I had them listed as local pickup only but a guy from the east coast reached out to me and told me about this. I added shipping as an option and he initiated the transaction through the listing and the shipping came out dramatically less than what I had gotten going straight to UPS and FedEx websites.
The big downside is it requires you to connect Facebook (of all entities) with your bank account, since they mediate the transaction and pay you through direct deposit. I hadn't done that up to that point, but I didn't have high hopes for selling a set of YJ soft uppers that only fit Bestop or JCR trail doors so I went for it so I could finally unload them.