buying apple watch from fb marketplace

Hello xljunitolx,

Welcome to the Apple Support Communities. I'll be happy to explain what to do before purchasing.

• If they are referring to paying off the Apple Watch as a whole, that is most likely financing. It should not affect any functions of the Apple Watch.
• If this is through a plan from a carrier, they would need to make sure the cellular plan is removed and cancelled. Some carriers might prevent devices being used on their network. It's best to reach out to the carrier directly for more information. Here's a link to show how to remove the cellular plan: Set up cellular on your Apple Watch

The best thing to do is to have this resource handy when looking into Apple Watch purchases: What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else

Have a great day.