the $1 cards aren't necessarily a scam depending on the villager being sold, although i would always recommend looking into the seller and their feedback beforehand. on ebay, cards can go for as little as 99p in my experience depending on the popularity/aesthetic of the villager, so it's not wildly uncommon.
as for packs only containing NPC cards, i don't think that's a thing? but you can probably buy 'lots' made up of just NPC cards secondhand. pretty sure official, unopened packs are all random otherwise.
Also have to watch if cards are the official real amiibos, or fake ones. If it says fan-made, nfc, or anything of the sort it's not an official amiibo.
It's not possible to get packs of NPC cards. People resell individual NPC cards on eBay and Mercari, but it seems they have gotten more expensive since HHP came out. Prior to HHP, NPC cards didn't have much use, but now they are quite valuable.