The number of scam artists using the site would go down, because it's more difficult to be anonymous when money changes hands.
Only slightly more difficult. The reality is that stolen credit card numbers are easily available to anyone who really wants to find and buy them (in bulk) and so using "ability to pay money" as a criteria for detecting non-scammers is weak. That's precisely because what scammers are good at is illegal acts, like getting stolen credit card numbers.