I looked at something like that myself at one time, but these are fairly pricey. You might as well go with the Roleez that definitely works well in soft sand based on all the reports I have seen, for that kind of money.
The other thing to consider would be an inexpensive, fairly wide pneumatic tire that you could let the air out to increase the footprint in really soft stuff. That is what works best if you have a 4WD vehicle and often drive in soft sand. The Roleez wheels can be purchased individually as I recall, so check one of the kayak/canoe mags for ads.
Also check the DIY forum on KFS--there are several guys who have built their own carts, and some have gone with Roleez wheels, as I recall.
Also check out this site for some good ideas on how to build your own cart:
Also--remember the "Big Wheel" kids tricycles? They had really wide plastic wheels, and although not pneumatic, they work well in soft sand also because of the huge footprint. You might look around your neighborhood to see if anyone has one of these babies sitting around. Or check out your local Goodwill store or thrift shop to see if they have a Big Wheel, or maybe even the wheels by themselves. Good luck.