Let's see if I can apply some physiological and biochemical explanations here…
Disclaimer: Oda's One Piece world is NOT intended for much logic and it's been months since I had a proper biology course. Hence, any loop holes are to be, er, blinked upon and forgiven. Yes?
As we all know, alteration of the phenotype is very well related to the alteration in the genomic level. The Devil Fruits may play a very important part in evolution of OP verse, if one could be as bold to speculate that it's an inherited ability. Suppose that the modified genes are to be truncated in the meiosis, or further mutations may happen to hide their expressions, then there's another possibility that the Devil Fruit ability is not inherited. Subject is open for discussion.
Luffy's Rubber Body:
-The Gomu Gomu Fruit may alter the mechanical properties of Luffy's overall cells, including his osteoblasts and osteocytes, and extracellular matrix. It's unclear how far the alteration may go to various cells, though. If it were the case of a truly elastic material, any transient load subjected to the rubber will not deform the material after it's lifted. However, Luffy can break his skin and bleed. Those symptoms indicate that it's possible for his cutaneous and subcutaneous layers to be infiltrated until a capillary or a vessel breaks.
In conclusion, the degree of elasticity in his various cells all over the body are not the same. In addition, Luffy needs momentum to use his elastic power in a fight, generated by his muscles. To pack power and speed into one long rubber requires a hell lot of momentum, hence you can imagine just how strong Luffy actually is.
Buggy's Mystery Parts:
Buggy is able to separate any parts of his body consciously.
Buggy is able to feel pain inflicted in any of those separated parts.
Those separated parts are alive and fresh, even though they've been separated for a long period of time.
Buggy is able to re-connect his parts consciously.
He does not bleed when separated.
I've heard of a phenomenon where an electron miles away can influence the conduct of another electron. This may be one of the many explanations of Buggy's mystery. But I will take the more traditional approach, based on the body's ingenious neural system. The body uses neurotransmitter and electric current to respond to stimuli. In Buggy's case, I'd have to say that his body optimizes the electrical system to include the currents in the space/medium.
Why doesn't he bleed? Perhaps an extraordinarily fast case of blood coagulation?
Again, subject is open to discussion.