I handle support just through Gmail. Volume (sales and support) is very low, so I’ve had no need for a more complex system.
I have had very little of the “how do you install the theme” level of questions. I can only recall one time where I granted a refund for someone who didn’t even know what WordPress was. But the rest of the support includes a full range.
Hearing some of the horror stories from ThemeForest customers, I would guess that my customers are not particularly demanding and maybe come to it with less expectation of “customize all the things”. That said, “no bloat” is part of my sales pitch, so I may attract a certain customer. I did have one person complain that it wasn’t like “other themes where everything is just drag and drop”. That was a simple refund, though.
The minimal support load is probably the greatest attraction of going independent. Your sales and marketing pitch can cater to the crowd you’re interested in supporting.