I synced my old library to test what happens. Now I want to start my current library from zero base. For this, I want to reset my web library... There are about 5,000 items, and it's too large to delete every item one by one.
If you've configured syncing, your web library and your Zotero are kept in sync, changes done in one are reflected in the other.
If you want to remove all items in Zotero and start from scratch, you can select all items (cmd/ctrl+a) and remove these items (and later empty trash). Assuming you've configured syncing, these changes will propagate to web library.
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Could you explain what are you trying to achieve so we can try help you better?
Now I want to start my current library from zero base.
For this, I want to reset my web library...
There are about 5,000 items, and it's too large to delete every item one by one.
If you want to remove all items in Zotero and start from scratch, you can select all items (cmd/ctrl+a) and remove these items (and later empty trash). Assuming you've configured syncing, these changes will propagate to web library.