Your FBA stock can sell on eBay too. You list the item on eBay from ScanLister, and when it sells there, Amazon picks, packs and ships the eBay order from your FBA inventory. ScanLister places the order with Amazon for you and sends the tracking back to eBay. You never touch the item.
Amazon charges its own fee for shipping orders that come from other sites, separate from your FBA fees. Check Amazon's current rates for your items before pricing them on eBay.
FBA items reach eBay from the Inventory screen, not from a listing session. Open an FBA item and click Add to eBay (multi-channel), or tick several and use the button in the selection bar. The note beside the button confirms that Amazon will ship the eBay orders.

The item waits on the eBay Listing tab, tagged Shipped by Amazon. Its quantity is your current FBA stock. Edit the title, price and other details as you would for any item, then list.

The fee screen tells you how many FBA items are in the batch. They use an eBay shipping policy called "Shipped by Amazon (2 day handling)", which ScanLister sets up on your eBay account for you, based on your default shipping policy.

In Orders, these sales are tagged Shipped by Amazon. There is no label to buy for them; the Ship panel says Amazon is shipping the order.
If an order cannot go to Amazon as it is, for example the buyer has not paid yet or the address is incomplete, it waits, and its row in Orders shows a Needs you button that explains why and what you can do. If a buyer cancels on eBay, ScanLister cancels the Amazon order too, as long as Amazon has not started picking it.
Amazon owns FBA quantities, so the eBay quantity follows your FBA stock automatically. You cannot type a different number. When your FBA stock runs out, the eBay listing goes to zero, and it comes back when you restock.
An Amazon-shipped eBay listing keeps selling through Amazon until you end it. To stop, end the eBay listing from the Inventory screen. See Managing Your eBay Listings.
If you listed it on eBay yourself, merge the eBay listing with the FBA item. The eBay listing then switches to Amazon shipping, and its quantity follows your FBA stock from then on. See Merging listings for the steps.