I think selling something you bought for a profit is fine if you are still selling it for a fairly reasonable price. That's just basic economics. Yes, even the reasonable price part. You're more likely to make SOME kind of money if your price is competitive, anyway.
Selling something for an ABSURDLY high price is greedy and definitely questionable, but with us all being human beings, it's not like I don't understand that. However, you are less likely to sell this way, and your buying of the product might just turn into you holding onto it forever, when someone else could have bought it at retail price.
Going out of your way to buy something as part of some mission to get revenge on a fictional character is where I draw the line, especially when you group real people (i.e. the fans of the character) into it.
However, there is one point I want to bring up in relation to Amiibo reselling - the difference in value between currencies, and the difference in prices of Amiibo at retail.
You see, Amiibos are more expensive in Europe compared to the US. The cheapest I can get Amiibos is Littlewoods Ireland, at €12 ($13.60). However, they only sell 5 characters - Mario, DK, Peach, Link, and Pikachu. So they won't be my regular Amiibo seller. And if I wanted to re-sell, they don't have any Amiibos worth reselling.
Next cheapest place is Argos. They sell everyone but Wave 1 at €13.99 ($15.86). Wave 1 sells for €14.99 ($16.99), and Smyths Ireland also sells all Amiibos at that price. Finally, GameStop Ireland sells them at €16.99 ($19.26).
I once bought Pit and Wii Fit Trainer for my friend in New York. The former was a Christmas present for him, the latter was a Christmas present he could not find to give to his friend. He only pays for WFT and the shipping, but for the purpose of the point I am making, I will pretend he paid for Pit too.
Pit was purchased in Smyths for €14.99, and WFT in Argos for €13.99 (before the price for Wave 1 went up there). Overall, €28.98 if my math isn't rusty. Since he lives in the US though, there are shipping fees. Total cost of shipping was €7.25. So now the overall total comes to €36.22.
How much is that in USD? $41.05, for just these two Amiibos. So, that's the LOWEST I could have sold those for without making a loss. It sounds like an absurd price to you in the US, but it wouldn't be me scalping - selling them for $41.05 would simply make me a return. No profit. If I just wanted to make small profit of say, €10, then the price becomes €46.22. Or $52.39.
So yeah, not everyone selling Amiibos for a high price on eBay is doing it for insane profit. Some people might have high prices because that's just how currency exchange works out. Nintendo, for some strange reason, decided to sell Amiibos for a higher price outside of the US (or maybe it was just the retailers), and instead of trying to adjust things to how the currency values differed. Ideally, everywhere would sell them for €12 over here, as that is very close to being on par with the US MSRP.