If you use a marketplace such as Themeforest, try to see if the developer takes care of the support and updates the theme.
I've started to use iThemes Builder, and it's quickly becoming my number one recommendation. Elegant Themes seems to be a good vendor with fair pricing. I'm not a customer, but I've tweaked their themes on my friends' sites.
I would not recommend using a theme that does not have a framework. You need something that has regular security updates. If you aren't willing to pay for a professional theme go with a free theme from woothemes. I have had numerous clients come to me after purchasing a theme from ThemeForest that was eventually hacked. Child Themes are the way to go.