To delete many Sell documents at once, select the Sell button in the header menu.
By selecting the Sell button, a new page will open up with a list of all your Company's sell documents.
Select the documents you wish to delete.
On the top right hand side of your screen, to the left of the export button, is a smaller button marked with an ellipsis "...".
When you select the ellipsis "..." button, a drop down list will appear with a single option "delete selected".
Once you click this button, all the selected documents will be deleted.