Tener,
Looks like you got some great replies. If the box itself is in good shape, no reason not to buy it if it's what you like. The price suggestions are in line in my opinion.
Like it has been said, redo the caster mounts either with the manufactures frames mentioned above or building your own plates that'll spread the weight out.
I built some for my older Listas. The main thing you want to do is spread the weight out front to back and the sidewall. The ones I made are about 6" wide 1/4" plate, mounted so the sidewall and the corners are bearing on the plate. I welded studs on the plate for the casters to mount to.
I posted on my shop thread building these.
Let's us know how you made out.