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.
I think the point is it would become much less worthwhile to spam the site, at least with 'simple' spam. Stolen credit card numbers are easily available but using them just to spam Craigslist is probably a poor return on investment. Ironically, more complex scams could become more worthwhile if users start to show a higher level of trust in ads because they've been paid for.
You are probably right that increased trust because of payments could make the scams work better, but bear in mind that a typical stolen credit card can be bought for a few cents. They are almost free.
You are almost right:
Merchants that readily deliver goods to wherever carders (credit card scammers) ask them to -- go out of business.
Ultimately it's merchant's responsibility to make sure credit card is legitimate.
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.