As Orlove noted, barter may occur in commercial economies, usually during periods of monetary crisis. During such a crisis, currency may be in short supply, or highly devalued through hyperinflation. In such cases, money ceases to be the universal medium of exchange or standard of value. Money may be in such short supply that it becomes an item of barter itself rather than the means of exchange. Barter may also occur when people cannot afford to keep money (as when hyperinflation quickly devalues it).
While some people may balk at an online community for finding babysitters, I know several parents who say it’s actually very difficult to find a sitter they can trust. BabysitterExchange started in 2000 as a babysitting co-op, and it has since expanded to the point that members use it to reserve time when they just have some errands run, need help tutoring their kids, or want a temporary house-sitter.