Just report the income in the business income section, and report what you paid for the inventory you actually sold in 2019, in the Inventory/Cost of Goods Sold (COGS) section. The program takes care of it.
Lets say you have 100 widgets that you paid $1 each for, and you sold them for $0.50 each. The business income will show $50
Meanwhile in COGS you will show that you paid $100 for that inventory you actually sold. The program will work it and "know" that you have a $50 loss. Not a big deal.