I did the same, but all of mine were good (so far) (LJ12A3-4-ZBX (NPN NO 4mm) proximity switches. After jogging end on into a switch, I bought a replacement from a certain Company in Devon for £17.95 (It still stings!!). I changed the approach to sideways on. I suppose it depends on who makes them as to how reliable they are.

Dean would be obliged to replace any faulty switch he supplied, so his mark up is to cover that. Also he has his reputation to think of, so he will have found a reliable supplier of quality switches () and I would expect the switches are tested before dispatch...