I wasn't sure what to expect from this post. I originally though he was talking about a tool cart.
I see now that it's a very good idea, though I seriously question if people would be willing to pay $500. You basically need 2 carts. one for the rear and one for the front to steer. The rear one should be fixed wheels like this

The front cart should be very similar only should have a swivel table on top that will allow you to turn the front wheels, plus it should have a steering/pulling handle. This way you can use the same axles and wheels for front and back and nearly identical carts, only adding the swivel table and handle to the front cart. Perhaps it may be more economical to sell cart kits where people do the welding themselves? Or even better if it's a bolt together design. You would be able to save the customer money on shipping by shipping it in pieces or being able to pack it more compactly, and saving the customer the assembly cost. It would be much easier to make the pieces than the whole assembly too. Also consider using angle iron bolted together for the top hoop. this would allow the customer to drill holes and bolt 2x wood on top so as not to marr the timbers. This could be a suggestion to the customer as it would be too easy for them to do this as opposed of having 4 pieces of 2X shipped across the country with the kit. The bolting holes could be pre punched in the angle iron at little cost. Can you tell I'm a product designer/engineer?