I also thought about having people send me their own garments to dye. I wouldn't be a manufacturer at all, really, since the shirt is manufactured already and already owned by the person, so I wouldn't be selling it to them. They'd be paying me to decorate, same as someone would pay a decorator to decorate a room or house - they already own it, they're just improving it, or paying an expert to do it for them.
That would be my rationale. Such a "service" doesn't appear to me to be the same as "selling" or "manufacturing" the way I do now.
That said, I fully expect the CPSC to use the same common sense they are in requiring second-hand items to be certified before being sold, and applying the law retroactively. :P I'd want on OK in writing from them before I'd breathe easy about it - although dyeing other people's stuff I might be willing to consider.