What does Ebay have to do with an online store?

At first glance: nothing. And yet there is potential for synergies here.

On the one hand, you have your store where you manage your orders. Or the orders flow from the store into your merchandise management system.
On the other hand, you have a lot of sales and ratings via Ebay, often even more than in the store.

How do you get this together ideally?

Use your store as a product supplier. You don't need to create your products for Ebay again. List them directly from your Shopadmin to Ebay. This way you have your stock centrally in view and managed. Incoming orders flow from Ebay to your store and from there to your Wawi.
The whole thing works with Magnalister, a nice provider from Frankfurt/M. with very good support. The first tests are free. After that there are monthly fees.

Use Ebay as your rating provider. Many traders have been power sellers on Ebay for a long time before they open their first online store. Show in your store that you have years of experience and that you are not a start-up;
With a Shopvote plugin you can display Ebay reviews in your store.
And with a so called Rich Snippet Code you can even send this data to Google, so that (hopefully) Google shows yellow rating stars under your search results.

And: of course you have the chance to turn your Ebay customers into store customers.