I've been wondering the same thing. Seeing that I'm a college student, I'm not made of money, so getting new lenses for my 8x10 will take a while. The only real problem that I can see is that there are some people out there that WON'T return your lens, even when they're supposed to.
the other problem(possibly) is getting the proper lensboard issue(un-mounting, re-mounting, etc...) this could be a tedious situation.
I'd think that the best way(at least for my piece of mind) would be to take some sort of deposit(like renting from a camera store) that would be refunded when its been returned.
unless you've done prior business with someone(through selling or buying) I'd be wary of someone saying, "ya, I'd like to swap for a while". They may just part off with your beloved lens.
and insuring the package both ways of course for full value

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-Dan