What should I keep in mind while Seller financing my property?

Hi, 

I am considering seller financing one of the property I am selling. I got it off HUD auction and it has too many use restrictions for any lender to lend on it; and buyer's loan application is getting rejected from many places. So, I am considering seller financing it.

I am wondering what should I keep in mind/check.  I am already asking for 

1. 20% down payment to cover selling expenses, tax and potential foreclosure if we ever get there

2. Buyer's credit report and financial statement from their business. 

What else should I ask for?

Second question is; what kind of documentation should be done in this situation? 

Thanks

--Tejas.