Deal of the century, when it saves your life.
The battery can be replaced on EPIRBS, they get tested and are good to go again for another 5 or so years.
That said, you can probably get a new one for ~US$1K, - so whats it cost to repower and test/ recertify/re register the old unit he's offering you to a new owner?
You might find he's done his sums and figures it's cheaper to sell the old one - and buy a new one and the difference is close to what the above messing around to make the old one good again will be.
You need to do your homework is all.
406 Epirb is like American Express Gold Credit Card - you should never leave home without it!
Cheers!