Windows 7 Enterprise licence policy

Greetings!

I want to clarify some points concerning Enterprise activation:

1. Can be Enterprise activated on multiple PCs using one key?
2. How can I check Enterprise key validity?

Some private seller offers me Enterprise key which is eligible for installation on 2 PCs.

• Is that possible and were there such precedents?
• How can I be assured in his truthfullness?
• Is there some Microsoft service which allows checking key validity (had it been activated already or not)?

Thank you for the answer.

For an Enterprise Key to be valid, you MUST purchase it from an authorised reseller

In other word there is no clear way to check its validity before activation? Maybe seller activated the key 3 times, then the company bankrupted (common case in our region) and he doesn't know what to do with the rest of activations.