I'm obviously very self serving in saying this, but I think the best place to buy a used back is from an entity whose core competency, reputation, and experience is in evaluating, selling, and supporting digital backs.
At any given time we (Digital Transitions) usually have more
pre-owned Phase/Mamiya-Leaf digital backs than eBay, gDPI, and LL combined. The price is often a bit lower on eBay or from a private buyer but so is the risk. We inspect every back in a thorough test process (and we know exactly what and how to test since we have a long history of supporting and selling the same backs). We're happy to provide you pre-purchase testing or evaluation of your own. We also always include a warranty, support, and training (if desired).
I bought my Pontiac Solstice pre-owned from a dealer for the same reason. I'm sure I could have spent hours online and found someone selling one privately for a little less, but my buying process was stress free and straight forward and I had a much higher confidence in the result.
My suggestion, in the very least, is to find a dealer you know you can trust and let them know what you're looking for. Our inventory turn over is quite high, so even when we don't have a particular back on Monday we may have one come in on Tuesday (usually as part of upgrades). There is really no harm in doing this.