[delecti]

I'm not going to say it's "totally fine", but surely you can see why it's reasonable for them to detect the situation as suspicious. It's unclear whether OP was using a new or established account, but in either case they suddenly started adding multiple credit cards, each only for a single transaction. It's also unclear how many ebooks OP was trying to buy, but "so I started buying them one by one" certainly implies that OP added at least a half-dozen separate new cards in the span of a a few minutes.