There is quite a secondary market for onewheels. There are people that buy them 6 at a time and part them out. I have seen specials and know of friends that have purchased XRs new from stores for $1300. People that know, will buy them and sell them on Marketplace and Ebay for a profit.
From a merchant's POV if it doesn't sell in x amount of time they will let them go at cost or even a loss. People look for smaller board shops where a $2000 robot skateboard may not move compared to the rest of the sub $500 inventory. Then buy it and market it to the right audience for "top dollar".
I bought mine used. When I got it I emailed FM to ask how long I had on the warranty. If it is brand new they will be able to tell you , if it isn't they will be able to tell you when it was purchased for the first time.
As far as I know there is no serial number registry or database for stolen one wheels. though there prolly should be.