I have several that I got off a ebay too.
I have not had any problems with them but I also keep a very close eye on my animals.
The sticker on the front of them really sucks and is off a good bit but that is what thermometers are for.
I have one running my QT tanks, one running my hatchling rack with back heat and two running backup to my VEs.
After a couple days of tinkering I have yet to adjust any of them in close to a year.