Why do people sell if they have equity?

[Jeremy England]

So I'm doing alot of reading on rei.  Books articles and what not.  I see the strategy of driving for dollars and direct mails and the like.  So for a deal to be good the owner must have equity right?  Unless you short sell and I'm reading that is difficult and not exactly a good deal.  So the question is why would someone with equity sell to me below what the property is worth?  

I get that they are in disrepair. But say it's a 100k ARV and they have 50k in equity. Why would someone sell to me for say 55k? Why not just market the place with a realtor for 70 or 80k and call it a day?

I understand some properties would be unable to be sold via a conventional lender due to the state of disrepair and I get that some sellers just want to get what they can and dump the place.  But is that common?  

Where are you getting most of your deals? Distressed sellers or REO's or auction? Just soaking it all in, so thanks