My girlfriend was selling at a craft fair recently, and I bought her a nice pair of earrings from another vendor who used Intuit's GoPayment system, and honestly I probably wouldn't have remembered to come back once I had cash (if I'd even gone to get any). In that sort of environment, the standard is 'cash only', so adding Visa/MC to that is a bonus, not a drawback, and in my case it made her a $60 sale she wouldn't have otherwise had.

This is especially true for higher-ticket items like silver jewellery, where I'm not likely to have $60 kicking around in my pocket, but I'll have no problem putting $60 on my Visa. Personally, I also find it hard to part with $60 in cash (more tangible) vs. putting it on my Visa or debit card.

For actual businesses (like bigger food carts), a proper 3G bank terminal is probably the appropriate response, but in places where people aren't likely to expect any card use at all (smaller food carts, craft fairs, yard sales), having Square (or similar) is a net positive.