-Can you sell games you make?
YUP
-Can it make multiplayer games that work over the internet?
YUP
-How many polygons can a 3d model have before it starts to
make the game run slowly?
DEPENDS ON SYSTEM REALLY
CPU GPU RAM OS
-How much better is C++?
C++ has been around for a very long time and is multifaceted
language adapatable for many coding problems.
Its a very long winded language tho compared to DB and is way more flexible but DBP allows u to write (1) line of code
to produce a cube and not (100) lines
-Do all BDPro games require DirectX8?
YUP its written inside this language unless u some how capture the DX data and port to OGL or something with your own core engine per say.