You didn't say what type of organization your business is. I assume you are reporting it as a sole proprietorship on Schedule C. If not, the answer might be different.
In the business expenses section I would put the seller fees under "commissions and fees." In TurboTax it might just be called "commissions," but on your actual tax return TurboTax will put it on the line for "commissions and fees," Schedule C line 10.