Active participles describe the doer; passive participles describe the entity affected by the action.
VerbActive participlePassive participle
كَتَبَكَاتِبٌمَكْتُوبٌ
قَالَقَائِلٌمَقُولٌ
بَاعَبَائِعٌمَبِيعٌ
خَافَخَائِفٌمَخُوفٌ
دَعَادَاعٍمَدْعُوٌّ
رَمَىرَامٍمَرْمِيٌّ
ذَبَّذَابٌّمَذْبُوبٌ
The hamzah in قَائِلٌ and بَائِعٌ comes from hollow-root alteration, not a root hamzah. Dāʿin involves final weakening, whereas madʿuwwun reflects assimilation.