Mixing a yard in garbage bags doesn't sound like a one man job to me. I'd go with Jinks's idea of a HF mixer. Locally, a concrete plant that's one of the largest in the state has a $100 delivery charge for anything less than 4 yards, plus a $25 fuel surcharge at current diesel prices.
By the time you add buggy rental, you'd have $500+ in a yard. Then you have the issue of scheduling since a one yard load isn't gonna be high on the plant's list of priorities yet you need to be able to coordinate the truck and the buggy or possibly end up with another half day rental on the buggy. A mixer allows you to do the job on your own schedule, and when you're done you have the mixer which is worth something as opposed to the same amount of money sunk in delivery and rental charges.
Re the u-cart option, there was one of those places here that started up several years ago. In a fairly short time, contractors wouldn't touch the stuff, so they were entirely dependent on homeowners for sales and they folded after a couple years. The concept is good, but the execution not so much.