Hi!
I'm quite new and I didn't understand a thing about UV.
Skelly Shield, according to wiki, came with a maximum freeze defense after crafting.
So how does a frezze defense UV apply here? Is it completely useless (since the freeze defence is already maxed), somewhat valuable (since UV stack a little over maximum defense) or high valuable (since you will have an incredible ultra mega super high freeze defence)?
In other words: if I want to sell a skelly shield with UV, will I get more money from a high freeze defense UV or from any other high defense UV?
It still applies. Even though the Skelly already has the bar all the way over (take all bars with a grain of salt, they are not accurate). You can benefit all the way up to Maximum even if you already had full bars. In this case Freeze UV on your shield means it can soak more Freeze damage before breaking. Whether or not you get frozen when hit by freezing damage depends on how much Freeze protection you have on your armor not your shield.
As far as selling it goes how much a person values added freeze is more personal. Most people I think tend to look for shields that carry UVs that broaden the shields protection by adding resistances it did not already have. That said a Skelly line shield with UV Fire Maximum is still not going to soak fire damage as well as an equivalent Owlite line shield. If you want to sell it or do a price check try starting a tread over in the Bazaar. I tend to switch between 3 different shields depending on the stratum anyway so I personally would value a stacked UV like that higher then a broadened UV, because I like to tune to maximize my defense for specific damage types.
Its not going to fetch as high a price as gear that is tuned for FSC though. Currently the only T3 boss run is against Normal/Fire/Shadow/Elemental damage types, so these would likely fetch the best prices.
I really like my Sinister Skelly Shield and it is my go-to shield for Fiend and Undead stratums.