Casters could be mounted on piece of tubing that bolted to the upright. That would be fairly easy to fabricate. I've never done that before but it might be interesting to do. If nothing else it would give me an easy way to push the car that I'm not working on out of the way.

I'm not sure on pricing. I know what people would want to pay for a pair but I'm not Harbor Freight. I build stuff by hand in low qty so it costs a lot more. Honestly, I probably can't make these things cheap enough to sell them. I got tired of people bitching about how much the motor dollies cost so I stopped making them which tells me that I probably shouldn't try making these. I do know someone who is better set up to make stuff like this so I'll pitch the idea over to him.