That's probably true, and I thought of that. I've bought a lot of things from such sources on ebay, sometimes I'm pleased, but sometimes I'm not. At least ebay is good on refunds.
The problem with generic smoke & CO alarms is they can't offer support, answer questions, include testing data, and such. Not the sort of safety device I want to try out & see. At least with smoke alarms you can test with some smoldering cotton rope in the driveway with garden hose handy, but CO alarms demand such minute accuracy - difficult to test. Then there is the sensor drift problem. It's relatively new and economical technology, but maybe with imitators.