Measuring a cart

I have someone wanting to trade a cart for my saddle. How do I measure the cart to make sure if will fit my mule?

I came across a website that showed this. Sorry I can't recall it but it's out there.

American Driving Society

There is a lot more than this table to buying a cart. Make sure the wheels are in excellent condition. I would love to see photos of the cart.

Thank you very much! They didn't send me any pictures of the cart yet, so I don't even know if I am interested or not.