I buy frequently from EBAY. To cull your fear if the units you buy are any good, I usually order in quantities of 5 to 10. In total over the last 5 years I've ordered about 50 Nano's. Not one of the them were faulty when I received them. Yes, I blew a few up myself doing stupid stuff, but then I had many spare units because I ordered so many at one go.
I've ordered Nano's, Pro Mini, ESP8266, ESP32, STM32 and a new Nano Clone LGT8F328P. Admittedly the latter is the only one I haven't tested yet. Just got it a while ago and haven't had the time to put it through it's paces yet.