As you probably know, any used unit is a gamble unless the seller has tested it in a car and viewed live data on a digital scanner. Returning one from overseas may not be worth the shipping expense.
Not to scare anyone, but I bought a used one last year (as a spare for the hoard, lol) and tested it in my E420. Worked great. Left it in for a couple weeks, and 100 miles later it died... fully dead, stuck at 1.20 volts / 4kg reading. This was after the return window had closed of course. Sigh... win some, lose some...