the logic of this seems to be that if the salt water didn't hurt it and its now near worthless, give it a good soak in fresh water, redry and keep fingers crossed. In for a penny etc...take the first cover off first!

Another tack if you just happened to want another Sigma lens thats twice as expensive, you could try to pin the trade in value (they like to collect old lenses for spares) then ask for the new lens at cost; they like to keep customers sweet and it costs them nowt