Tell them that you are not ICE, but you need to know who is living in your house and verify that they do not have criminal records. 

Trust but verify, verify.

He still keeps access to the online account, assuming he has one (if not, get him one), so he can verify monthly payments are being made.

Then when you do an inspection you can verify what the seller said about the condition. 

Should I have a real estate attorney verify the legality of the BP lease agreement or am I safe to just go ahead and use it?

Also verify the zoning as part of the usual due-diligence on a park to make sure the park is still at least grandfathered in as an MHP.

How I can even verify that the seller has this agreement in place and has the authority to sell the property? 

@Jonathan Anderson Like most others mentioned, looking at their tax returns is one way of verifying their income.

Tenant placement AND management is the best pricing, and since I have to deal with who we pick....especially now....I'm screening and verifying everything.Gotta have some skin in it, in my opinion.

@Conner Daniel, first let me preface this by letting it be known that I am not a tax professional so you should always verify tax discussions with a qualified tax professional.