Selling Printed Clothing Online

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Has anyone got experience with selling their own printed clothing designs online, specifially through a wholesaler or company that prints on demand? I think I have a good idea for a gap in the market (a niche market, admittedly) but I want to aim for high quality cotton if possible. I know sites like Redbubble use a lower quality feeling material probably due to margin, but I would be happy to tighten up the margin for the right supplier.

Any tips or pitfalls would be appreciated.