If I sell a hand gun that I installed an Apex trigger kit per the instructions, (has worked 100% for me), can I be sued if some numb nuts blow his toe off?  Should I remove the Apex parts and sell the gun with its original components?.

Hey, what could go wrong?

Yes, you can and whether you win or not, you lose. Many will yell you not to sweat it, but you're asking the question so decide how much risk you want to take. At a minimum, advise the buyer with written, provable notice.

Other options include selling it to a dealer to help insulate yourself and getting less for it is pretty cheap insurance. You can also get a release from the buyer

If you modify your bicycle, motorcycle, car, or any other item and it malfunctions are you responsible?

Perhaps, IF you sold it with any sort of warranty or guarantee of workmanship.  The way to avoid responsibility would be to sell "as is" and make no claims.

Yes, actually, you do run that risk. Just had a friend finish settlement on exactly that related to motorcycle modifications.  Jer crashed due to speed and poor skills and blamed the seller based on the modifications. Even tire size changes that aren't manufacturer approved can be a source of claims on motorcycles and trucks. Sucks, but it is real.

I'm not saying the sky is falling on this, but the risk is there and there are things you can do to protect against it.