If it's a good price (the only reason to buy a used PLB imo), take possession, reregister and then send it to the manufacturer for a check-up and new battery(ies). My neighbor who took his small (for me) but well built sport-fishing boat to sea (blue water just west of here) received my first PLB after I upgraded to my ACR ResQLink. He reregistered and trusted me that it had not been activated. By now, the batteries have timed out.

Not being the trusting type, I'd send it in. $.02