How you handle it depends on their behavior, your laws, and your skills.

It could be an easy fix like this or a bigger problem depending on the factors at play (are the joists accessible, will it mess anything else up if you just jack it up and reinforce, is there water damage or structural damage to the foundation causing the sag instead of just old joists, anything else going on like a supporting wall or posts under the floor, wiring or plumbing runs involved etc.).

The primary determinant (beyond the tax implications) is whether the lender will underwrite and permit the borrower to close in a company or require the individuals to be reflected on title. 

You can use a DSCR loan (you can find lenders by using the "Find a Lender" tab on BP) and not have to worry about a tax income or W-2s, the underwriting is done based off of the property's cash flow.

Depending on the zip code you might also check with Wells Fargo to see if they offer the down payment assistance grants in that area.Could be the buyer is just trying to save cash, so counter back with no closing costs.   

Concentrate on knowing local market dynamics, financial planning, legal and tax implications, and long-term objectives.

definitely. i have got the details from them for their existing landlord. however, that name does not match the owner on the city tax office property records. and they have confirmed that they provided the landlord's details, not the property manager's details.I believe that should match or it is possible to not match?

Get pre-approved and submit offers on whatever property you like.

It all depends on what sort of deal you are thinking.  

If it wasn't fraud but simple incompetence (and he lost the money), then that isn't a crime and your options are more limited.Your legal contract (and/or the state laws the entity was incorporated in...depending on where) most likely requires the sponsor to share books and records with you (so you can at least know what happened).