Get your taxes done using TurboTax

No, you can't do that. A negative number won't result if you do the computation correctly. The way the computation works is this:

           Beginning Inventory (what you purchased before the beginning of 2018 that you didn't sell in 2017)

PLUS: Purchases (less items withdrawn for personal use)  For this they mean subtract the items you withdrew for personal use from the purchases you made this year. Not what you took from last year's inventory. That part will come later.

MINUS: Ending Inventory (what you have left at the end of the year.

EQUALS: Cost of Goods Sold for the year. 

If you pull more items from inventory than you purchased in 2018, you'll be pulling from your beginning inventory, so you need to reduce your beginning inventory by the amount you pulled. You'll have to check that your beginning inventory is different from last year's ending inventory, and attach a statement as to why.  You can do all that in TurboTax.