if we decide to be EXTREMELY charitable, the seller could have mistaken MacOS 14 Sonoma (which ALL Apple-Silicon M-series Macs can run) with 10.14 and sloppily looked up 10.14 Mojave by accident.
But users here are finding that mistake glaringly obvious, and it may not bode well for the overall knowledge of the Seller. It could be a STRONG danger sign. Since I am not an Apple employee, I can blurt it out: "This might be stolen!!"
The problem for you (as a buyer) is that if the computer is actually running MacOS 14 Sonoma when you get it, it has NOT been properly prepared for a new owner.
When properly prepared, all the old owner information AND the old version of macOS (which is tagged with the Apple-ID of the person who dowloaded it) would have been removed, and the computer would have the "factory-original" MacOS installed (tagged only with computer serial number). It would NOT be set up. It would be waiting for you (as new owner) to power up and automatically set up from scratch.
If the OLD owner information is still present in any form, the Mac can not be updated AT ALL. It is simply not yours to modify. Tthis is the process:
What to do before you sell, give away, trade in, or recycle your Mac - Apple Support
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The oldest MacOS that was 'shipped in the box' with the earliest M1 Mac was Big Sur 11.0.1 earlier versions, as already observed, were Intel 64-bit code, ONLY (no apple-Silicon code at all).