The difference is probably in components. The kart in your link appears to have aluminum wheels, standard pills instead of independent caster/camber in the front, inexpensive seat, and probably cheap aftermarket axle. I’m just guessing at these items, but that’s usually where a vendor will make concessions to reduce the expense for an entry level kart. Even if all of that is true, it’s still a nice deal at $3500 shipped.
The Righetti Ridolfi components I’ve bought were all very nice. They’ve been around for a long time, so support shouldn’t be a problem.