Scupper Cart
This is a scupper cart. It is a set of wheels that fits in the scupper holes of the kayak and lets one person wheel it around like a wagon. The big advantage of this sort of cart is that when you get to the water, you can pull it out, flip it over, and stick it in the top of the kayak. Then off you go - no need to double back to put the cart away. Also, you'll have it if you get somewhere that you might want to portage.
The cart pictured above is pretty enormous. I shortened the uprights by about two inches to reduce the lift-over, and took about 7 inches off the axle to make it fit inside the outline of the boat. The foam on the uprights also had to come off, and I added a light cam-buckle strap to secure it.
If you have a 'sit-inside' kayak, then you'll need to drill a set of holes in the bottom of the boat to use one of these carts. You might want to look for something else.