These racks are in great condition with no issues. They come with straps for 2 kayaks, but they can hold up to 4 kayaks if you buy more straps. They mount to oval/aero cross bars, which are commonly found on modern vehicles now. They also come with a lock and key to secure them to your vehicle with no worries.
I kayak often and used to use two sets of foldable J-hook kayak racks but when I switched to a kayak stacker like this, it was clearly a much better option. Since the kayaks stand more verticle while strapped to the stacker, rain doesn't get into the kayaks as easily and I didn't need to empty water from the cockpit as often. There's also less wind resistance going down the road and the kayaks were always very secure. This was our spare set of stackers we used for a second car on river running trips but we no longer need them.