To be honest I don't know much about card valuation as I've always just been a buyer. The furthest I've ever gotten is just to check EBay and see what people there think the market can bare for a particular card.
For example, I checked a couple top sponsored search results on EBay and a "single" specific North American card seems to go for:
Lottie 017 - $4-6 USD
Phil 057 - $9-20 USD
Eunice 081 - $7-20 USD
GOOD GRIEF!!! I don't know why Phil and Eunice can command such a high price on EBay, are they super popular amongst people who want to show off islands or something?
Of course, these are EBay "Buy Now" prices, so I don't know how real they are (maybe the real price is lower and all those are sold out, or maybe EBay didn't want to show me the lowest prices?).
Finally, you've got the JPN version of the cards. Someone correct me if I'm wrong but the game itself doesn't care it's the JPN version, Amiibo Cards are region free so functionally i's the same. If people are buying to have a complete NA collection instead of just to play the game with that'll probably put them off and drive the price down?
On top of that I honestly don't know how I'd even begin to sell the cards I DO have if I did decide to try that.