No money down duplex purchase! Woohoo! What am I missing?

[Tim Richardson]

I had a conversation with an off-market seller of a duplex near Salem, Oregon. Due to the purchase/rent ratio not being conducive to putting 20% down, I offered to purchase with them carrying 30%. All I pay is some closing costs and run with it (tenants already in place). 

What fine print should I be sure to include in my "official" offer other than the standard things like buyer to do due diligence, must be satisfied with inspections, etc.?