The OTTO Marketplace API does not offer the possibility to retrieve the stock.
This means: If you change stock levels in the webshop, magnalister can only synchronize stock levels to OTTO that have been uploaded via magnalister. Therefore, remove all products not listed via magnalister and upload them again to OTTO via magnalister.
However, when importing orders from OTTO to the shop, the stock in the shop will be reduced. A prerequisite for this is that the SKU on OTTO and in the webshop are identical.