If I purchased and used a refurbished iPhone from eBay - will Apple buy it in Trade-In when I want to buy a latest iPhone ?

According to the Apple GiveBack program,

"You can trade in Apple and third-party devices. Many Apple and third-party devices are eligible for a trade-in refund or an Apple Store Gift Card. And all Apple devices are eligible for recycling. Either way, hand us your used devices and we will handle them responsibly."

There is a FAQ at the bottom of the GiveBack page: Trade in with Apple GiveBack - Apple

If the 'refurbishment' involved physically opening it up and / or replacing parts by an unauthorized 3rd party, Apple won't touch it for trade in. They will recycle it, but you're not going to get any money for it.

Be very careful. If it was sold by anyone other than Apple, odds are it is NOT Apple refurbished.

Be careful about buying iOS devices you cannot physically check first. If it has Find My set or activation lock engaged then it is useless to you and I doubt Apple would accept it for trade-in either.

The only source of Apple OEM refurbished products is through Apple itself, on their online refurbished store. Anyone else selling refurbished is either selling a used Apple refurb they purchased from Apple (is that what the seller is advertising?), or it is third party refurbished (e.g. Amazon refurbished iPhones are refurbished by Amazon or their contractor, not Apple) which may well void it from further consideration by Apple if internal parts were replaced.