Apple watch

Hi

Apple Watch Series 1 can be used in conjunction with a compatible model of iPhone with a service plan from any carrier:

Set up your Apple Watch - Apple Support

Be sure to remove Activation Lock before selling your Apple Watch:

About Activation Lock on your Apple Watch - Apple Support

Amberc2975 wrote:

I have AT&T. If I sold my Apple Watch Series one to a person who has Verizon will it work?

Yes, Series 1 watches are not tied to a carrier.

Be sure to follow the advice listed in: About Activation Lock on your Apple Watch - Apple Support

No models of Apple Watch are tied to any carrier.

Whether or not the Apple Watch will work with any given buyer's iPhone will depend on their iPhone model and on the installed version of iOS.

See my previous reply in respect of your specific question.