Well, persumably the stab wound would be seperating part of Buggy's insides from other parts of his insides. His body would seperate as a result, as if he had been "chopped." Of course, this ais all speculation, but it seems to be one of the feasible reasons why, despite receiving no other types of wounds, that Buggy has not yet shown any types of wounds from bladed weapons.
Actually, That question is answered in episode 424. Luffy is seen throwing down Buggys feet, sending them toward sharp spikes. Those spikes "goes through" his feat and have the same effect as if they were to cut him. If the object is sharp enough, it will result ina "cutting" effect. ^^