i know people do not like undercutters but is it against subeta rules

Not at all if you want to sell something for 1sP and its worth 10mill no one can stop you, its your right as a user to pick what ever price you want

And who decides what items worth what anyways?

However, if you have an item in your shop that is selling for less than normally, the official Subeta Team Account occassionally buy it from your shop, instead of another user. I'm not sure if this still happens, but it might not work the way you plan.