There are two main factors which can contribute to a negative account balance:
Unless otherwise specified in your Amazon Services Business Solutions Agreement, your account is subject to a EUR 39.00 monthly subscription fee. This fee is assessed independent of your inventory or sales volume. You will incur a negative balance if your sales do not exceed your subscription fee (along with any other negative charges).
Product refunds can also contribute to a negative balance. When a product is purchased and refunded during the same settlement period, the negative impact on your account is minimal because Amazon Services will only retain the lesser of EUR 5.00 or 20% of the original commission fee in order to cover transactional costs. These fees are generally offset by other sales. However, if you issue a refund for a product for which you have already received payment during an earlier settlement period, Amazon Services will charge you the full amount of the refund in order to recover the previous payment.
Your settlement report will provide a detailed view of all credits and charges applied to your account. The negative balance at the end of a settlement period will appear in the next settlement period as the beginning balance, along with the related Amazon charge. Learn more about downloading and reading your Settlement Report.