In my experience, many Craigslist buyers (especially of high-value / high-margin items) tend to be rather afraid of buying things on Craigslist, and would be unlikely to swipe their credit card with the seller's phone. Naturally this tendency will vary from market to market, and SF would probably be a good place to experiment with using Square in CL transactions.

I refused to deal credit card based payments b'c of chargebacks risks. With chargebacks, the credit card company will always side with their customers, often leaving the seller with no recourse to retrieve the money or the item sold.

In addition, there are enough scammers lurking around on craigslist that I didn't want to expose myself to those kinds of risks.

I was also buying/reselling items with fairly low-margins. Credit card processing companies e.g. square, paypal, etc. will only eat away at whatever little profit I was making at the end of the day.