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Hi HarmonyBliss02,
The good news is, based on your description and if those two discs are legitimate, either can work with your PC. The more important thing is a Windows 8.1 Pro license/product key along with suitable installation media.
However, I must note in advance that I cannot confirm which one is “better” for you, especially since they are third-party eBay listings. I couldn't directly verify the authenticity and licensing status of those particular copies.
Still, if we based solely on their descriptions, the Retail copy is the more conventional choice for an individual home user, while the OEM-oriented option is cheaper and suitable if you don't have plans to move the PC.
- System Builder/OEM copy generally has more restrictive licensing: It is intended to remain with the PC on which the OEM license is first installed. So once installed, it can only be used on this PC.
- Retail copy gives you more flexibility: Depends on the type, you can transfer the license to a new device in the future, making this more suitable if transfer rights are important to you.
Choosing which is entirely up to you. But there is also an important point here (in case you didn't know): Windows 8.1 reached end of support on January 10, 2023. Microsoft no longer provides software updates, security updates, or support for it. Microsoft recommends moving to a currently supported version of Windows instead.
If your PC doesn't meet the requirements for newer version, consider preparing it fully before continuing with Windows 8.1. When buying either of the options above or another source you find, make sure the seller is actually providing a genuine, valid license/product key, rather than only the DVD.