That's a very real problem. In fact you're completely right, the seller CAN modify the installer as he pleases and it can do virtually anything after that (whether or not he actually did is another history).
To detect such tampering, the usual procedure is to compare to a well-known source that you can trust it's clean. That implies downloading the Windows ISO image from Microsoft and comparing it. But that defeats the purpose, since you already have the original disc, why bother with testing another one?
So I would say that it doesn't matters. Just download the original disc from Microsoft and forget about the other one. You can always make backups of that to have handy for another time.
I am thinking that the only way a person can know he is safe is to use restore disks that come with a PC OR buy a windows 7 install from New Egg or something like that.
Not exactly. The
ONLY way to get a known-clean installer is to get it directly from the producer himself, MS in this case (that applies to every software out there, not just Windows). The so-called "recovery disks" are just an image of the out-of-box state of pre-built computers, and in many cases are known to be tampered with unwanted programs or even malware in some cases, they're not safe at all.
As for that "New Egg", that's just..... a private seller, not a privileged redistributor. The disks they provide aren't inherently any safer that those got form anywhere else, which return us to the very same starting point of this thred.
You can not legally buy a COA key only.
You buy a legal Microsoft Windows 7 and it comes with a matching legal COA key.
Quite the opposite. When you buy software (any software) whay you're really buying is not the program, but the license to use it. Though that license you get a serial to activate the program for unlimited use. But it's that license (and accompanying serial, "COA" or whatever) what is actually bought, the install disk in itself has no value, and is even freely distributed sometimes. MS itself had freely available Windows 7 install disks some years ago, which can only be activated though purchase of respective licenses.