I buy on eBay all the time, but never anything that ages over time (batteries, etc.). I always check the seller’s reviews, and never from a seller that has only a few reviews.
What can be wrong with a used SmartThings MultiSensor?
(1) it’s broken, Pay with PayPal, then you’ve got the eBay return policy for goods received broken, and the PayPal return policy as a backup. I’ve used both when I have received broken goods. If the seller refunds promptly (has happened every time except one), then the seller is honest and just shipped something not well tested. If seller doesn’t refund, and eBay doesn’t back you up, then Payapal is good about clawing back your money. I had one scammer, from whom I had ordered a TV for my wife’s daughter, who apparently shipped something else to an accomplice in my ZIP code. eBay refused to refund, said seller provided proof of delivery, but PayPal refunded when I provided manufacturer spec sheet showing the weight of the TV, and the UPS package “delivered” (to the accomplice) only weighed a few ounces.
(2) it needs a firmware update. If you’ve got a SmartThings hub, you can update the firmware. If not, ask the seller to update before shipping, and don’t purchase until seller agrees.
(3) it needs a new battery. You should expect this, and adjust what you are willing to pay.
(4) the adhesive for the magnet or the base of the MultiSensor needs replacing. You should expect this, and adjust what you are willing to pay. 3M sells sheets of adhesive.
Yep, it’s supply and demand, and, as for excessively high prices, well, P.T. Barnum had something to say about one being born every minute.