Yes, carts are great for one person! There are so many different carts available now, and most of them are really good. I use a standard 2-wheel cart just for getting the harp around.
I also have a six-wheel dolly designed for stair use. It is great for curbs and short flights of steps, but for a large flight of steps you are still pulling the entire weight of the harp and I find it a little scary. I prefer to take the harp off the dolly and have someone help me carry it up large flights. I’m not very strong and some of my friends can handle large flights fine, so I think a lot just depends on you.