That model fender roller originally was built with one set screw in the crank arm. The newer model came with two set screws and the crank shaft had two flat spots on it, to keep it from slipping. I took a photo of an extra crank arm I have, to illustrate the modification on the newer machine. notice the two set screws.
I originally had one of the older models and the arm did tend to come loose, however, the roller was used heavily before I bought it and I rolled out at least 200 fenders on it before I started having to stop and tighten it up. These are great machines. I have had no problems with the newer machine coming loose and it looks like the machine being sold is the newer one. I used to stomp a fender flat at a show and then roll it out perfect, as a demonstration. Amazing machine and it is being sold at a fair price. Cover the top roller with electrical tape and one can roll out a badly dented painted, or chrome fender, without disturbing the original paint. It's a fun machine to own and quickly pays for itself.