The problem with that method is that it won't really hit 100% of all sectors that are writable. I'd guess especially not in CF cards, and possibly not in SD cards since there is generally a small amount of storage space set aside as spare area and for wear-leveling, which means it's not accessible via normal LBA parameters, but the controller can still read/write to it. You probably can trick the controller into reading from those spare sectors with some effort, which means you could still recover some data.
And I'm kinda surprised someone will buy some older cards, especially if a few of them give you an error in your 5d3. If you're honestly that worried, then just thrown them out. You aren't going to get much money for them, are you? If it's anything like $20-30 and you're really worried, I'd just toss them, save yourself somewhat of a headache. If that's _per_ card, then maybe it's worth it, since after 3 or 4 cards you can get some nice, big, shiney new cards. But that begs the question, why is the buyer not willing to shell out for some decent new cards? They aren't that expensive in the 8-16GB range.