Rookie question: If cashflow is so great, why is it being sold?

[Aaron Rosenberg]

I haven't made my first deal yet, but I found this AMAZING house.  Okay it's an awful house, nearly 100 years old.  But I can pay cash for it and it already has 100% occupancy.  I'd have said cash back in 4 years...OK 5  6 7 years because I'll have to fix a few things here and there, but then I will have my money back and cashflow literally beyond anything I'd imagined before.  We're talking FOUR figures here, and not just over that $1000 threshold either.  (I played Robert Kiyosaki's game; a property cashflows like $200.)  

So...why is this even being sold?  Why would an owner divest himself of such a figurative gold mine?  

I guess I'm really asking you what the hidden traps are in such deals.  An internet ad isn't going to tell me what to watch out for.  So, other than "why is the seller selling," what should I ask the realtor, and what traps should I be weary of?  

(If you're interested, after the showing I can post what I learned here for further discussion and educational purposes.)