How to get fitting loose?

[BlueOvalFitter]

After you remove it install a full port 1/4 turn ball valve.

 

[demarpaint]

From the looks....I'm NOT a plumber, But play one at home, The faucet extension is Soldered in & they didn't use the 1/2" NPT connection.

One solution is to cut the faucet off at the extension & Sweat on a 1/2" Female Copper Solder x 1/2" Female NPT Adaptor, Least you won't need to Sweat a connection next time.

That's how I would do it.

 

[Snagglefoot]

I just rebuilt a no-name unit. I couldn’t find a kit so I purchased an entire valve and pulled the insides out and stripped off the seals I needed. Although the seals are identical the course threads to lock the unit in place were a different pitch. ( ignore the fact that the unit I bought was shorter and cheaper). Even if they were the same length they wouldn’t be interchangeable. My original one was 10 years old. If you are dealing with a recognizable brand you might be able to change out the entire inside unit.

 

[Snagglefoot]

Here are some shots. Notice one has about three threads per inch and the other about 4 threads per inch. They are not interchangeable but the seals are. The rod in the top photo already had the front seal removed. That seal is an ordinary faucet seal.