First of all, a dolly is one of those tools in your garage that you need to have a low profile. If you get a large model, it might not fit well in your shed. However, you can pick from different models on Amazon, and some have a lower profile than others. So, your first consideration should be how much space you have for storing the dolly.
Then, consider the weight of the dolly. It should be lightweight so that you can push it around with ease. Another thing is the weight limit accepted. Combine these criteria with the low profile requirement for a good selection. A good warranty should also be on your list.