If you think it was sold on eBay it should be reasonably easy to track down the transaction history as long as it was fairly recent.
Browse to the Go-Karts (Recreational) category and then add the following filters
Item Location: Within 25 Miles of (Your Zip Code)
Show Only: Sold Listings
Once you have that you’d be best served to contact your local police department and probably eBay also. Even if you aren’t able to find a record of the sale online I would file a report with the police.
It always helps to have proof of ownership when you file a report with the police. Whether its receipts, service records or even photos the more you have the better.
Luckily I’ve never had a bike/kart/car stolen, but a couple of years ago someone snagged my guitar pedal board. I found the pedals at a pawn shop and was able to get them all back because I had boxes they came in and was able to match up part/serial numbers. As a bonus they caught the guy and recovered some other goods he had pilfered from my neighbors.
Best of luck trying to get it back.