I think a mini-skein swap would be a great idea. I am not big in knitting socks, so I don't have too many different sock-yarn-leftovers and most of my hexipuff are kinda the same colours, not that much variation really.
I'd rather swap sekins. If it was a finished puff swap I'd be worried that my gauge is a bit off from swap-partners and people might be different in if they stuff them and how much they stuff them with and so on. And postage would be cheaper as well.
How about having more than one swap partners? Maybe 3-5 miniskeins to 3-5 partners?