And once more for the umpteenth time, buying parts on ebay is always a crap shoot.
before you throw them out, though, try wiring one into a real circuit and see if the contacts don't just burn themselves clean. A couple of sparky on/off cycles might just clear away whatever film there is from the manufacturing process.
Good call. I just built a dim bulb tester which comes in handy for testing the switches at mains voltage (which is 120VAC 60Hz here). A 100 watt light bulb in series with the live wire serves to limit the current and is a fairly heavy load. The tube amp I'm building is a small one and I don't expect it to consume that much power so this should function as a good test case to see how well these switches hold up.
As I said before, the resistance would change every time you switched it on and off. Before testing it was anywhere from 1 to 20 ohms. For the first test I left the switch in the on position and used a power strip to switch the mains. After disconnecting the power I again measured the resistance in the on position and it came out to around 0.3 ohms. I toggled the switch on and off a few times disconnected and the resistance jumped around again between 1 and 8 ohms.
For the second test, I reconnected everything and toggled the switch itself while it was pulling current hoping, as Enzo suggested, that arcing inside the switch would burn the contacts clean. That seemed to be the case. When I disconnected the mains and measured the resistance it seemed to be a pretty consistent 0.3 - 0.4 ohms when toggling on and off.
That's still not a great value, about 30 times what was quoted in the datasheet, but not as bad as the readings I was getting suggested. The switch did not heat up at all. Maybe I can salvage these things after all but I think more testing is still in order before I trust these things in my amp circuit. It didn't occur to me to measure the voltage across the switch while it was on so I'll probably do that next. That way I can calculate the resistance of the contacts while it's in a live circuit.