They periodically need to be recalibrated regardless, so there is no guarantee a used meter is exactly reading. I find my meters need recalibration about every ten years, and made it a habit to
keep an unused one to check the others. Richard Ritter or Quality Light Metric in Hollywood can do
the work. They need to be calibrated across the range, and not just at the middle. And commercial
gray cards are rarely good enough - you'd be amazed at how much they vary, even within the
same brand! DLSR meters also function in a wholly different manner, so you're likely to get off on a
wrong foot altogether is using one of them as a reference.