"Have any of you done that when you purchased a machine used? how would I even do that? it seems like so much effort. would I ask for the serial numbers when negotiating before even seeing the equipment???"
I've purchased several high value items on Craigs List, Never had a problem. Get a Bill of Sale with the serial # written on it if you buy. I seriously doubt you could track down any liens on mowers. Now if the guy selling a mower wanted to meet you in a parking lot (not at his residence) & sell you a mower, I'd say the chances are it would be a stolen item. Plenty of good deals out there on CL these days.