One risk of buying a used system on eBay is that it may have been configured with something other than the default installer code. If the seller doesn't know what it is or doesn't tell you, you're out of luck. So ask before bidding.
If it works when you receive it, it's likely to keep working for some time. Think of the previous owner as having done an extended burn-in for you.
The thing that you will be losing out on vs buying a new panel is technical support from an authorized seller.
If you already own an M1 and have the RP software, you'll be all set for using it with another panel. If you don't have a copy, then I suspect Elk will authorize you for a download even though you bought it used.
