So with my massive experience (not), I've found the following works to clear the categories and the products. Just make a good DB backup first then you can revert back. Be careful with the delete pictures command as it remove them all, including the store logo, sliders etc. It's better to remove them one by one as you need then reseeding to the last picture you actually want to keep, for example, my first 23 pictures are logos, sliders etc so use DBCC CHECKIDENT (Picture, RESEED, 23)
This essentially gives you a clean store.
DELETE FROM ShoppingCartItem
DELETE FROM ReturnRequest
DELETE FROM OrderNote
DELETE FROM GiftCard
DELETE FROM OrderItem
DELETE FROM RecurringPaymentHistory
DELETE FROM RewardPointsHistory
DELETE FROM [Order]
DELETE FROM RelatedProduct
DELETE FROM ProductAttributeCombination
DELETE FROM ProductAttributeValue
DELETE FROM Product_ProductAttribute_Mapping
DELETE FROM ProductAttribute
DELETE FROM ProductReviewHelpfulness
DELETE FROM ProductReview
DELETE FROM Product_Picture_Mapping
DELETE FROM Picture
DELETE FROM Product_ProductTag_Mapping
DELETE FROM ProductTag
DELETE FROM Product_Category_Mapping
DELETE FROM Product
DELETE FROM Category
DELETE FROM Manufacturer
DBCC CHECKIDENT (ShoppingCartItem, RESEED, 0)
DBCC CHECKIDENT (ReturnRequest, RESEED, 0)
DBCC CHECKIDENT (OrderNote, RESEED, 0)
DBCC CHECKIDENT (OrderItem, RESEED, 0)
DBCC CHECKIDENT (RecurringPaymentHistory, RESEED, 0)
DBCC CHECKIDENT (RewardPointsHistory, RESEED, 0)
DBCC CHECKIDENT ([Order], RESEED, 0)
DBCC CHECKIDENT (RelatedProduct, RESEED, 0)
DBCC CHECKIDENT (Product_ProductAttribute_Mapping, RESEED, 0)
DBCC CHECKIDENT (ProductAttributeCombination, RESEED, 0)
DBCC CHECKIDENT (ProductAttributeValue, RESEED, 0)
DBCC CHECKIDENT (ProductAttribute, RESEED, 0)
DBCC CHECKIDENT (ProductReviewHelpfulness, RESEED, 0)
DBCC CHECKIDENT (ProductReview, RESEED, 0)
DBCC CHECKIDENT (Product_Picture_Mapping, RESEED, 0)
DBCC CHECKIDENT (Picture, RESEED, 0)
DBCC CHECKIDENT (ProductTag, RESEED, 0)
DBCC CHECKIDENT (Product_Category_Mapping, RESEED, 0)
DBCC CHECKIDENT (Product, RESEED, 0)
DBCC CHECKIDENT (Category, RESEED, 0)
DBCC CHECKIDENT (Manufacturer, RESEED, 0)