It would depend on what I think the sales revenue will be. If I think the monthly overall sales revenue will be $20,000 or more, then I would negotiate between 5-10% of sales. If I think it is going to be less than $20,000 per month, then I would negotiate between 10%-50%, depending on how much less. Obviously, I would want a higher percentage, the lower the expected sales. The bare minimum for me to consider selling the product at all would be $4,000 in monthly sales revenue - if I think it will have less than $4,000 in monthly sales, I would want a reasonable percentage (maybe 10-15%) but also charge cash sales fees between $2,000 to $5,000 depending on how much work is involved in doing the competitive product research, keyword and listing research, and whether I will have access to recommending product changes based on negative reviews, or bundling options based on reviews, questions answered, and Amazon's suggested 'also bought.'