YOu don't. COGS is what you "actually paid" for the inventory. Being that 2016 was your first year dealing with COGS, your beginning of year (BOY) inventory *must* be zero.
BOY Inventory - what *you* paid for the inventory in your physical possession on Jan 1 of the tax year. If this is your first year, the BOY inventory will *always* be zero.
EOY (End of Year) Inventory - what *you* paid for the inventory in your physical possession on Dec 31 of the tax year.
Cost of Goods Sold (COGS) - what *you* paid for the inventory you actually sold during the tax year.
The BOY inventory for a tax year must be exactly the same as the EOY inventory from the previous year. If it's not, then you'll have some explaining to do to the IRS, and it won't be pleasant. That is why, in your first year of dealing with inventory, your BOY Inventory will *always* be zero. It has to match the EOY inventory of the prior year, which can "only" be zero, since you had no inventory at the end of the previous year.
Note that what you paid for the inventory can only be deducted in the tax year you actually sold it. It does not matter in what year you purchased it.
Lets say you had $1000 of inventory that at one time in the past was your personal property and you did "in fact" pay $1000 for that inventory in the past, before you even though of ever having a business. THen you open your business in 2016 and actually sell 1000 worth of inventory. (You ordered another $1000 of inventory in 2016, and sold it that same year.) Here's how the numbers would look.
BOY Inventory - $0
EOY Inventory - $1000
COGS - $1000
The above indicates that you started the year with no inventory. Then what you paid for the inventory actually sold, was $1000 dollars. The fact that you paid that $1000 for that inventory 15 years ago (and can prove it because you have receipts from 15 years ago.) doesn't matter. Then you ended the year with $1000 of inventory in your physical possession.
