Sorry for the delay in answering your question, but it is a difficult one to answer without knowing more about you and your circumstances.

Under normal circumstances (2020 being anything but normal), the only parts you are likely to need are recoil springs and magazine springs and used springs of these two types are probably not a great choice for replacement duties.

Other used springs can be reused if they are still in good shape, but that implies that the original springs are also NOT likely to be unserviceable.

Of the hard parts with any likelihood of becoming worn and needing replacement, only three come to mind (in reverse order of frequency):

Old style ejectors that rarely break.

Extractors that wear or become chipped.

Sear release levers that neither wear nor break, but are rendered unusable by slide and frame wear (probably the most common of the NON normal wear items to require replacement).

Old style ejectors are best replaced with new style parts if they fail.

Extractors are a fitted part so used is usually not an option (and the extractor spring would also be replaced at the same time so new on that, too).

The same goes for the sear release lever.
It, too is a fitted part and should also be new.

The final category would be parts broken or lost due to, shall we say, inattention...

These are most likely to be small parts (and used parts are OK), but inattention around firearms is never OK.

Full disclosure:

I have lost exactly one part in my lifetime association with guns.

That would be the detent plunger in the windage adjusting screw in the S&W 2nd/3rd gen adjustable rear sight.

That tiny plunger is undoubtedly the smallest part I have ever encountered on any firearm.

I have worked on quite a few of those "big eared" sights and have only the one loss.

That is not to say I have never had a part roll away on the bench or even hit the floor, it's just that I have been able to find all of the others.

Therefore, it is nearly impossible to predict which parts would be best to keep on hand.

I keep lots of parts, but I am an "experimenter".

Considering that most of these S&W 1st/2nd/3rd gen parts are still commonly available, but nearing the end of production, it's probably not a bad idea to stock up as they are available, if for nothing else than "trade fodder".

That's my free advice and worth every penny.

John