I just thought I give everyone interested in building a cart a heads up about these wheels. I'm using a pair. They are under $12/each including shipping.
They don't have any steel parts to rust, and they are pretty wide. If you needed to cover a lot of ground over soft sand, you could double them up as duallies. They have sets of four for under $28.
Here's the current e-bay auction for one wheel:
http://www.ebay.com/itm/Tire-4-10x3-...item1e7d337f99
SellerID is small-engine-deals.
Keywords to search on if that auction is dead: Tire, 4.10, 3.50, Tubed, "t410x35PLAS"
Looks like you can get them directly from the seller here cheaper for the list price, but I'm not sure what the shipping would be:
http://www.smallenginewarehouse.com/t410x35PLAS.html