Automatic checking is on from the start, and it sends what it finds to eBay, with safety limits: a big price jump is held for you to look at, a check that could not read enough pages sends nothing, and a limit stops too many changes going out at once. If you would rather watch first, open Settings and choose check only: it reads your suppliers and updates the figures without touching eBay, so you can see what it would do. Switch sending back on when you trust it.
When a sale comes in it appears on Orders: open it to see the supplier's page, the buyer's address in copy-ready lines (read live from eBay, never saved) and what you will keep. Order from your supplier, then paste the tracking number in and it goes straight onto the eBay order.
Your Overview shows what needs you, most urgent first. A backup is kept every day, and Export everything gives you one file that restores onto a fresh install. Leave Sellhelm open on your PC: automatic checks run while it is running.