Seller is asking for a hard cash transaction

Hello,

I have been presented with the opportunity to purchase a house from a friend. 

Her father has died and is looking to move the property as quick and fast as possible. She owns the property with two other family members. There is also no mortgage on the property. 

However, the sellers are looking for a hard cash transaction. I’m not sure what the reasoning is behind this though. My friend is a trustworthy person and have zero concerns in that regard. Lawyers would obviously be involved. 

Has anyone ever completed a transaction like this?

Can anyone lend advise?

Is there anything to watch out for? 

Red flags? 

IRS? 

Thanks