The reason the OTA update won't work is that you have clockworkmod rather than the stock HTC recovery (which is a consequence of the previous owner having rooted the device, or attempted to). With that you can flash custom ROMs, but cannot install official OTA updates (and if you want to do custom ROMs frankly most people recommend TWRP for the One rather than CWM).
So there are two options. One is to install a custom ROM, which you've said you don't want to do. The other is to return the phone to stock, unrooted status. The rest of this post assumes you want to do the second.
You can check the phone is unlocked by shutting down then rebooting while pressing the volume down button. You'll get a white screen with a short menu, and the text at the top will tell you whether it is locked or unlocked (S-On or S-Off would also be interesting, indeed just tell us what it says in the top couple of lines).
To return to stock you need to use a HTC ROM Update Utility (commonly abbreviated to RUU). This is a windows application. How you use it is:
1) relock the bootloader (if unlocked, which it probably is)
2) connect to a PC via usb
3) boot phone into the bootloader. You see "fastboot usb" near the top of the screen.
4) run the RUU .exe on the PC
Two complications here: how to relock the bootloader and how to choose the correct RUU?
Relocking the bootloader is not a problem for most, except you didn't actually unlock it yourself so will need to do some setting up first. Our
HTC One root thread has most of the information you need to do this. The first post in that thread gives you 3 links at the bottom. Use the first to download "adb" (and "fastboot" in the process). Follow the instructions at the top of the second post to set this up, going as far as the "fastboot devices" command. If that works, you can then use the command "fastboot oem lock" to relock the bootloader (don't worry about downloading stuff from HTCDev.com, as you are not unlocking the bootloader).
While you are there, use the command "fastboot getvar all" and copy the output (we may need this for the next bit).
As for choosing the RUU, these things will check the phone's original region and branding and the software version, and not run if they are incorrect. Software just has to be newer, or not older (an RUU is an
Update utility, and won't downgrade the software). For the rest, if you know that already then great, if not then use the command "fastboot getvar all" (see above) and post the results to us.
Once we know what RUU to look for, you can download the from
here.
Sorry about the hassle. Frankly I think it's a bit unfair to sell modified handsets without at least telling the buyer that the software has been modified.
