To scan barcodes and individual serial numbers on your iPad register, you need to first set up your barcodes and serial numbers at:
Products > Manage Products > Choose the product > Scroll down to Inventory > Track Stock Level > Choose Serialised > Save
For example, if you're selling an iPhone X, you can create the barcode for the product 'iPhone X' then add in the serial numbers for the individual phones like below.
You'll just need to make sure you have the barcodes for the serial numbers ready and ensure the barcode numbers match the serial numbers.
When you scan the product's barcode, your item will appear on the transaction column on the right.
Then, you would need to click on the 'Select Serial Number' pop up's Search Button and scan a second time to choose the serial number.