Is it safe to purchase food from the street markets in Naples?

Yes of course it is
Probably cleaner and safer than produce in a US grocery store

Is it safe to purchase from a local farm market or food truck in your home town?

No different in Naples; you need to decide.

Absolutely!
And much more fun.
But…in Italy, don’t touch any of the produce…the stall owner will do that for you.
It’s frowned upon to cherry-pick your own apples,etc.

wouldn't that be apple-pick?