You can run into smooth talking sales people who can repeat cliches with confidence but have little or no substance backing their assertions.

He spent it all on...well, I've been cautioned repeatedly not to mention ladies of questionable reputation and a certain white powdered substance with stimulant properties here on Bigger Pockets.Or give it to guru-boy and his shills.

Your proof of funds can be your most recent bank statements (acct. numbers censored out) - otherwise how would the seller know your offer actually has any substance to it? 

(IF) your sister-in-law is dealing with substance/drug abuse of some kind then you could encourage her to check into a rehab then you and your wife could help sort some things out while she’s getting help for herself. 

There was a milky substance spilled in the fridge, some dirty pans and dishes in the cabinets (I just put everything in and ran a load), my girls found an old spilled McDonalds cup under the nightstand upstairs and I pulled a water bottle cap from under the sheets when I crawled into bed last night.

Substance rules over form.

The stew is no longer seen as separate ingredients but rather as a pot of stew being its own form or substance collectively.

http://en.wikipedia.org/wiki/Analysis "Analysis is the process of breaking a complex topic or substance into smaller parts to gain a better understanding of it."

You are like a one person housing development agency, renewing homes and providing quality housing (not any easy task).If someone is just doing a surface flip (just cosmetic shine and not so much substance), I am not sure you help any one (save stimulating the economy some for cost of the floors, counters, and paint).

So the buyer wants to conduct some unspecified, unregulated test for a substance that occurs naturally in the environment - by a company for which no license/certification is required and for a substance for which no 'authority' has determined that some specific amount is 'unsafe'!