1) The full license you purchased, it this retail or OEM (system builder)? Windows license, (whether retail or system builder OEM) can be used at any given time on only ONE machine. Since you used it up on the old machine, it failed on the i7. My guess is you were doing online activation.
2) The keys on Ebay are frequently MSDN/ Technet keys being sold illegally. They work fine initially but once they are detected by MS (probably because a very large no. of people are using them, having bought from Ebay like yourself), they are blacklisted and the system turns non-genuine. So MS is right.
3) You can use the same physical disk to install on multiple machines but you need different product keys. Thats the 25 digit number on the dvd package. The product id is different- its a system generated value and does not need any user intervention. So dont concern yourself about product id, product key is all you need to know about.
4) As far as I can make out, your full version disk product key is still not being used. Use that on your main machine and activate by phone, if necessary explain what happened. Activate Windows 7 by Phone