Whether online forums such as Yahoo Groups like it or not, posting the same message across several groups is here to stay. Take for example this reference to cross-posting on the Shelbyville Times, Tennessee, where some author wants to advertise a Xbox 360 on a couple of freecycle groups. In this particular instance the blogger is questioning the merits of the post in question, asserting this actually may be a possible spammer. Whether they are or not, I would say it is common in announcement-styled groups – such as those that sell and give away items – often are subject to cross-posting.
I would also say that cross-posting can be very useful for items that have only a small audience. In the above example, I could see a great interest in an Xbox 360 and associated Wii devices. On the other hand if you want to dispose of, say a rather interesting, perhaps quirky bit of furniture then cross-posting is a critical way of finding that “one” person who likes that particular item and in whose home it will fit nicely.