Risks associated with this situation?

[George Moehlenhoff]

So I'm evaluating a property that has steam heat/window ac units. All the comps have forced air. All bids I have gotten are 10-15k, but I have a contact who is licensed and insured who could do it for maybe 50% of that price. Problem is it would have to be "under the table". What are your experiences with situations like this? What risks am I possibly assuming by not getting an I-9, etc etc.

Set me straight BP, thank you in advance!