Sell items
Go to Inventory → Items and tick the items you're selling.
Click Sell selected.
Sell this item opens with a ready-made invoice:
Check the Title, Date and Document number, and pick the Customer.
For each line, enter the Quantity. In stock shows what you have, and the Selling Price is filled in from the item.
Add a discount, tax, shipping or payment information if you need to.
Click Save as and choose Draft or Published.
The invoice appears in Revenue → Invoices, where you manage it like any other invoice.
Restock items
Tick the items you're buying more of and click Restock selected.
Restock this item opens with a ready-made bill. Pick the Vendor, enter each Quantity, and check the Cost price.
Click Save as and choose Draft or Published.
The bill appears in Spend → Bills. Record the payment there when you pay the vendor.
Tips
Ordering ahead? Use a purchase order. Publishing it restocks the items. See Create and publish purchase orders.
Stock looks wrong after a count? Use an adjustment, not a fake sale or restock. See Adjust stock and stock value.