You can transfer a license that you own from server to server.
You can't use a license that was on a server that was bought from previous owner of the server. The following is from the EULA for vSphere:
2. LICENSE GRANT.
2.1 Scope of License.Subject to the terms and conditions of this EULA, VMware grants You, during the License Term, a non-exclusive, non-transferable License to use the Software, in executable code form only, within the Territory, for Your internal operations in accordance with (a) the Documentation; (b) the License Type for which You have paid the applicable fees; (c) other applicable limitations set forth in the Order. The License to the Software is limited to the quantities specified in each applicable Order.