I bought my Apple Watch from someone and I’m trying to reset it and it keeps making me try to sign in under their email

The seller needed to remove the Activation Lock from the watch in their Apple ID before handing it off to you. Activation Lock prevents lost and stolen watches from being used by others.

You need to contact the seller and request that they remove the Lock. If they can't or they refuse, then you've been scammed and can never use the watch.

The seller can remove the Lock online following these instructions;

Remove  Watch Activation Lock -

Go to icloud.com and login

Open Find iPhone.

In the All Devices pulldown menu select the Watch.

In the resulting sheet that appears to the right choose erase Apple Watch. Select Next until the device is erased.

Then Remove the watch from the Apple ID.

This should reset the watch to the factory settings for you to setup as new and pair it to your iPhone.

M1 Mac mini, macOS 13.1; Watch, series 7 GPS/cellular, watchOS 9.2; iPad Pro 2nd Gen, iPadOS 16.2; iPhone 12 Pro, iOS 16.2; TV 4th Gen, tvOS 16.2; HomePods, audioOS 16.2 and Airport Express w/ Airplay 2

combatseabee wrote:

If you have an iMac or Apple laptop connect your watch by usb directly to your device and open iTunes. ITunes will sometimes just automatically open when you connect your device.

From there, you can reset as a new device.

Completely wrong. The OP purchased an Activation Locked Apple Watch. Connecting to their computer CANNOT remove Activation Lock without the Apple ID and Password used to set up the watch.

If you have an iMac or Apple laptop connect your watch by usb directly to your device and open iTunes. ITunes will sometimes just automatically open when you connect your device.

From there, you can reset as a new device.