I bought an iPhone from eBay and it’s asking me for an iCloud password I tried contacting the seller but he hasn’t replied over a week

If it was not advertised as iCloud or activation locked, and the seller is not responding to you, file a claim with eBay under their buyer protection guarantee.

The seller misrepresented it.

There is no way for you to bypass the activation lock.

Get a refund.

The iPhone is Activation Locked and will remain useless forever.

Turn off Find My iPhone Activation Lock - Apple Support

I am sorry to say I must agree with sberman.