Deciding how to group your product is much easier than deciding what to sell... so congratulate yourself on getting to this point in the process!!
There are a several different ways to group your product. Consider the following:
1. By Theme: Baby Girl, Holiday, Birthday, etc.
2. By Type: Cardstock, Pattern Paper, Ribbon, Embellishments, Thickers, Stickers, etc.
3. By Condition: New Product, Used Product, Scraps of paper
4. By Color: All shades of one color grouped together- this works well for cardstock and pattern paper.
4. By Kits: All remaining product and embellishments from a kit or paper line.
Find a good amount of space, like your kitchen table or counters, and begin sorting through your stash and grouping them in piles. You may decided to group them in a combination of ways... like your cardstock and pattern paper by color an your embellishments by type. You may even decide to make up your own themed kits by putting together a few pieces of cardstock, pattern paper, and adding a few embellishments that coordinate. If you have a lot of one product, consider making smaller groups. For example, if you have a lot of Halloween product, make up 2 or 3 kits. The idea is to present your product in a way that would appeal to the buyer.
Lastly... as you sort and group your product DO NOT create a 'return to scrap-stash' pile!