I'm not usually one for swaps, for the same reason other people have said- postal costs.
If you're thinking of starting this in the New Year, Ellie, maybe it would be worth a shout finding out what surface mail would cost for people who live in the Eastern states, at least.
It might be interesting to compare the time scales and prices - I STILL cannot figure out though, why I can get stuff to CA and WA in three days and it can take up to 10 days to get packages to ME and MA.
Think this is a brilliant swap idea!
All the Best