In other words, no evidence.

I've told Sellers that while I am not an attorney or real estate broker, but if they want to protect their position, I've seen others counter with language that states that the Buyer must produce evidence within perhaps 7-10 days that they have an End Buyer under contract that can be verified by a third party Escrow/Title company.

That's not evidence, but it matched my assumption your account was fake so I went with my gut.Sorry for the offense.

I also require all paperwork be signed by all parties, and they must show evidence that the utilities have been established in their name no later than the start date of the lease. 

There is a lot of concern (but not a lot of evidence) that the same thing may be starting to happen at scale in the Seattle region.If you are going to invest in individual real estate properties in ANY area whether it be local or foreign  please plan to be a good neighbor -  keep up the property,   use it the way it is intended to be used,  know all the local laws and follow them, rent it out at fair rates and terms to people you have background checked and will be good neighbors.  

I would not accept a German Shephard unless I had clear evidence they were capable of training it or paid someone to train it.3.

Somehow, you seem unhappy/uncertain, despite evident success?

I currently have a post here on BP asking if anyone knows what evidence a lender will require to qualify the property management experience.

This means, as evidence in court, the debt was paid on that date.Whenever you accept a check in person, you can insist upon the check being 'drawn' on the date received or refuse to accept it.I would create a form letter for receipt of all personally received payments (fill-in blanks for name, unit#, amount, date). 

This will not provide evidence that the vendor has a sales tax account, but it will show if he is registered with the state and you can call the state and see if they have a sales/use tax account.https://mycpa.cpa.state.tx.us/coa/