I have two Moto X (1st Gen.) that we would like to sell once we upgrade our phones and would like to know what other providers does the phone work with?
Your phone is only compatible with Republic Wireless. You can still sell it though. Actually, Republic legacy phones (such as your Moto X) tend to go for more than other carrier phones. You can sell it on a 3rd party site such as eBay or Swappa. See this document for information regarding selling a used device: Buying and Selling Used Republic Wireless Phones. Hope that helps!
Before you sell it remove any screen locks (pin) and factory reset it. Do not remove the SIM. It must stay with the phone. Mention in your ad that the phone works with the outstanding no-contract Republic Refund plan with unlimited talk and text for a mere $10.00 a month.