I ran into this problem today and couldn’t find a good answer online.
“How to delete all the products in OpenCart in one click when you have multiple pages of products”?
The Answer (remember to backup first):
1. Show more products per page on your products page.
(Go to System > Settings > Edit Your Store Settings > Click the Option Tab > Edit the “Default Items Per Page (Admin)”)
2. Go to Your Products Page in the Amin > Select All > Delete.
Enjoy Deleting Products!