My recently purchased laptop came with W8 installed and W7 discs provided. You really should have insisted on the same.
I reformatted my C:\ drive and re-installed from fresh.
Buying from Ebay is fraught with problems, but if your vendor has a long good history you should be safer. You might also be safer buying second-hand. But either way you take a gamble and you need to check what you buy and only accept a clearly official product. Bear in mind that modern fakes LOOK very good, and even deceive activation initially.
I suggest you study these links for further advice: