kobra:
First, only attempt this if you throughly understand what is contained in the DB tables, as there are several things installed in a fresh DB that you would not want to delete.
Backup you DB as the first step, this is a must!!
If you have phpMyadmin, go into your phpmyadmin app and load your DB. Select the tables that contain the information you want to clear out via the check boxes and then at the bottom there is a dropdown that is titled 'with selected'.
Dropping this down you will have several options and you want 'clear'. This will empty the contents retaining the structure.
Please proceed with caution...
This is what I ended up doing, it worked quite well.
delete from products where products_id > 129210;
delete from products_description where products_id > 129210;
delete from products_to_categories where products_id > 129210;
delete from categories where categories_id > 13383;
delete from categories_description where categories_id > 13383;
Now, when I do uploads to add new products I will be creating watermark products and categories to keep track of where things are in the DB..just in case.