You have to hire a hvac tech to recover the refrigerant. Then seal the system so there’s no chance of contamination. Then you need him again after set up to charge the system and test for leaks.
The refrigerant can be loaded into the compressor before being un hooked, but you have no way of knowing unless you were there for the un hook. Most mini splits are ran on 410a which is cheap and easy to get from multiple places. Optimally you would during set up have the lines vacuumed and pressure tested with nitrogen then refilled at the proper amount depending on line set length.
That being said I’ve un hooked and moved mine 3 times never having the lines vacuumed and adding a shot of 410a and it has ran fine for 2 years.
If you think you may have to move it in the future get a mr. cool with the diy fittings. If you want to buy once cry once go Mitsubishi. If you’re in cooler climates pay attention to the models with low ambient operating temps
Any used minis for sale are gonna be worked or they wouldnt be for sale unless you get real lucky and theres no way to tell until its installed so its a big gamble.Ive done many installs of new units and its fairly easy with a few tools,fittings,a nitrogen tank and a vacuum pump but unless the unit you buy was done right when uninstalled you are gonna need a pro to weigh in more refrigerant etc etc and theres still no guarantees its not just worn out or has a fried circuit board,only way to tell is to install and fire it up.
Best of luck!
Ive installed quite a few minis, just finished putting a Sanyo in, when I did my first one about 26 years ago there were only a few "quality" brands(mitsubishi,Friedeich,Lennox was #1). Now they are all about the same cheaply made stuff. I dont reccomend getting a used one unless its only a few hundred bucks or if it was installed but never ran. Most I install in grow rooms are cooling only(cheaper and less parts to malfunction).Myself I dont recover freon (unless I was at a company that required it)as ive been doing it long enough where when I started there was no such thing(ozone beware)