Usually when someone uses lots of different cards to make purchases on a website, they're testing stolen cards. 2-3 cards is fine, but if you have over ~5 cards in a day, it's over 90% chance that they're stolen. You simply don't run an ecommerce platform of any size without a system to automatically flag and block those transactions.

I haven't checked the logs, but I guarantee there have been people/bots trying to do card testing on our site in the past 24 hours. It's pretty nearly a non-stop barrage, so you have to auto-detect and auto-block.

Could be a person testing out stolen cards with a low-value purchase, to verify that the cards are legitimate. They then can use them elsewhere for much larger purchases.