Can I sell my iPhone 8 to Apple Store?

Apple's current trade-in offers and related details are available at https://www.apple.com/shop/trade-in

You can trade-in online, or at an Apple Store. (See the Frequently Asked Questions at the bottom of that page for details.)

In the link MrHoffman provided is a section for "Trade in and get Apple Gift Card credit — even without a purchase." This is the closest you'll get to Apple "buying" your iPhone 8. They won't give you cash for it. But they'll give you an Apple Gift Card, you can save and use towards a future purchase with Apple.