In early 2018 businesses bought more inventory, increasing GDP. What would happen if consumers didn't buy those goods?

If a company increases its final goods inventory level, that will increase the country's GDP even if the goods are not sold during the current period (they will be sold in future periods). Increases in inventory level are considered part of Investment component of the GDP.

If the inventories are not sold during the year (2018) and if they continue to be  unsold during part of 2019, that will negatively affect the GDP because fewer goods will be purchased to replenish these inventories and the market price of the goods will also decrease. The market price of goods is determined by the supply and demand. If the supply is much higher than the demand and inventories start to pile up, their price will decrease in order to increase the quantity demanded. If the volume of the goods in inventory is significant enough, this should lower the inflation rate.