Magnepan SMGa - what are they worth, best places to sell?

Generally speaking, unless the damage is severe, they only rattle at certain freqs and power levels(iow: once in a while).  Best way to test, is very slowly, with a sweep generator, at a decent amplitude.  Of course, the original owner(s), may have kept them out of the sun, in a controlled environment and they're fine.   Here's hoping!

The glue that secures the, "voicecoil" wiring to the diaphragms, on the early Maggies, often fails(age/sunlight). Be certain to test them, before selling. You may avoid an unhappy customer.  Around $400 sounds correct.   I gave mine to a friend, after we reglued them.