I’m thinking the reason why there are a lot of cheap NAV cards being sold is they have earlier generation maps. Once a new map update is ready, new cards go out with the new maps and they increment the letter at the end.
Dealers now have all the left over inventory to dump, but can’t do it officially, so throw them to eBay. At least, that’s the theory I’ve got.
Explains why you can never find current generation maps for much less than retail — not that you need to, because you can just update the old card as soon as you get it ... and continue to update it for 3 years after.