A new supplier has only offered a 10% margin. Should I accept this?

The initial margin offer from suppliers is often their first offer where they are just trying to see whether you would accept a very low margin. When a supplier provides their first margin offer / price list, they will not be surprised if you respond back to request better prices. 10% is a low margin and it reduces even more after payment processing fees and then including your marketing cost per acquisition. 

When you respond to your potential new supplier, you should explain that you need to be working with an improved margin to ensure that you can invest substantially into the marketing of their brand and products.

Explain that the better the margin they can provide, the more you can invest into the marketing of their products and therefore the more sales you will be able to generate for them.