You could auction them when they are in the campsite, providing you can actually get them to move into your town, and then get them to move out, but it would be better to just do it when you know they are leaving.
There's no guarantee that people would get them from the campsite-some people have said they have gotten them from other people's campsites, but I;ve seen no good evidence to suggest this. For me, it's a risky business. And even if it did work, I wouldn't charge for it ^^