There are two ways you can upload your sold books for removal to Biblio using FTP:
A) Upload a single file which contains both additions and deletions. Make sure there is a column or separate field in the file which denotes each record as sold or available (or otherwise distinguishes the action to take with the record).
B) Alternatively, you can send your records for deletion in a separate file. Simply make sure that the filename contains the word "deletes" somewhere in it. (For example, "upload-09022003-deletes.txt")