They adapted something that people were already using in great numbers, and provided a good interface for an extra hot flow without compromising what facebook was. (so that people would keep using facebook for this purpose, and not go to another platform with a dedicated workflow, e.g. craigslist or freecycle)

So they did actually provide value to end users here, and arguably captured some for themself too. I know the last n times I've opened facebook have been exclusively for buy/sell groups

The hard part is that companies don't hire creative thinkers, but specialists. So you let people try stuff until you think something good happened.