Welcome to TMO. Thanks for the post.
You need to be more precise regarding your OC'ing. If a device has been OC'd, it's only reasonable to declare the kernel you're using (i.e. Kernel Power version xx, where xx is probably a number between 46 - 49). As well as the max speeds used. The stock is 600MHz. Some devices might fail at 650MHz. Even though the standard warranty has expired, the device is still under a safety warranty (e.g. if it was undesputably found to have blown up your house you could get some help rebuilding). However, by OCing, even this warranty becomes void so OC'ing cannot, ever, be called "recommended".

Asking this much money for an OC'd device needs to be justified.

I should also ask:
- how many times a week do you plug a USB cable into the device?
- do you have h-e-n installed?
- - if so, how much do you connect usb devices?
- do each of the pc and charger cables still have the little nodules on the micro-usb?

Apologies if all that sounds a bit abrupt. N900s are not fault free and each device is quite different from the next so it's only reasonable to declare all the info you have on yours.