I worry this is the step in between illegal downloads and making anonymity illegal. If they can flag you for downloading unpaid content, it won't be long before they flag you for circumstantial patterns leading to that behavior such as using TOR.
Try being truly anonymous on the Internet these days. It's near impossible. Most online vendors won't accept pre-paid credit cards. Many email providers require you to provide a phone number or another email address as "verification".
Even posting on forums is difficult, because while Tor is decent for fetching data, some websites blacklist Tor IP addresses (or, even if they don't, there's a chance you may be sharing an IP address with someone who was banned from a forum).
Paypal, for one, makes no attempt to hide this: https://www.paypal-community.com/t5/About-My-Account/using-p...
While Paypal will accept it if you provide a Visa-verified billing address, not all will, and that would defeat the point of anonymity anyway.
Visa and AmEx also forbid international payments on prepaid cards, and most merchants nowadays require a verified billing address. (This wasn't the case as recently as 2009).
Remember that a prepaid card is not a credit card - there's no risk of you not paying, because they [Visa] already have your money and yet merchants refuse to take it. It's completely ridiculous.
(Semi-related: Do prepaid cards count as debit for purposes of the regulation on swipe fees?)
It's a problem with both parties, since both are known to refuse to process them.
> Is there any way for a merchant to know that a card is prepaid?
Yes
> Is there any way for a merchant to know that a card is prepaid?
No