We have some useful report that can help you in our point-of-sale system, although they are labeled magazine, they actually work for all products.
Go to the Main Menu / Stock Systems / Supply Report
The first step is to check the Over Supply Report.
Put 100% in the box.
Select the option to show all
Select option Returns
Now click Show Report
These are all the products you have received and never sold in the period chosen. I would suggest these are the first ones to cull.
Now if you need more room, select the Magazine Supply Report.
I suggest you do it by supplier because you will see an overall pattern by suppliers, and you find when you cull you need to see the pattern across a supplier's range.
Now select the supplier and period and then click Get Data.
Now on the reports, you will see a listing of the items sold by numbers. Note the bottom selling items.
Actually its surprising how much can be culled with little effect on sales.